That is not how it works in canon, Ezechiel357. It does not recast the spell repeatedly. It is cast once and then runs forever (assuming nothing else stops it). That is what those questions are in reference to.
Then I must admit I didn't understand what mr Love meant.
Never mind me.
Reading above, that's not the case, D:Sun, environmental trigger of sunrise/sunset is how the cost is calculated for determining the level and thus the amount of vis necessary, but doesn't describe how the item works. The item works constantly, without interruption and without recasting the effect once the effect is activated. If that's the case of how it truly works, then the item can be left anywhere for safety, while the effect continues to remain active.
And this is why Jonathan is asking good questions. 
Personally, I would have allowed Constant only with effects on the item itself. I would have cited "have a constant effect" as opposed to wording like "create," "sustain," etc. in place of "have." I believe that would avoid all the issues Jonathan brings up. But, alas, my choice here is not canon, clearly seen from the same group of effects in the necromancy section of HP as exposed the 2/day vs. truly constant interpretation.
It's not an effect being targeted, but an item generating an effect.
An item can be active or inactive. Controlling which state is in doesn't destroy the item --- hence ReVi.
I'm still interesting in someone directing me to a thread where this logic was hashed out. I think the arguments for and against will be fascinating considering that the one part of the puzzle that I DO have is the conclusion being 180 degrees diametrically opposed to the starting conditions.
But do devices even have a deactivation mechanism? IMHO no. They are activated and the effect runs the course of its duration.
And the main difference to me between using ReVi to suppress an effect or PeVi to destroy it is that the Rego method needs a duration, using Perdo is momentary. And, as Tellus mentioned, the Rego effect suppressing the device's effect is detectable ac active magic using PeVi, and can itself be suppressed or removed.
However a crafty and skilled investigator can also detect traces of magic, sensing residue of spells no longer present...but the traces quickly degenerate into negative magnitudes and are difficult to sense. HoH:TL:Guernicus has material on this.
Christian is right, assuming the device isn't maintaining concentration. The standard item fires off the spell and is done. It's like shooting a bullet. Imagine shooting a bullet from a sniper rifle at a distant target. Right afterward you dismantle (very quickly) the gun so the bullet will stop? If a mage casts a D: Sun spell, could you kill the mage to turn the spell off? That only works (assuming they don't have the Virtue/Flaw that stops their magic upon death) if the spell is D: Concentration. Similarly, that is how items that maintain concentration work and why they can be turned off, which is why I agreed that the method is valid with items with triggers to turn them off (i.e. items that maintain concentration).
No one will be able to do that. I had caught the 1/day MuVi effect and thought it was an error. I submitted that along with a whole bunch of other issues for that bit in Hermetic Projects. The response was that the 1/day was not an error, that my interpretation of a constant spell as firing off twice per day to keep itself going is incorrect. It actually just activates and runs forever. I can't say when the decision was made, just that that is how it was interpreted by the author of that section in Hermetic Projects and that that is how David Chart said it works.
You might want to read this part more strongly:
It's not ending and being replaced by an identical effect repeatedly. The device "has a truly constant effect."
So far, my understanding was that it looks like a continuous effect, with a seamless recasting - which for me was in line with the 2 casting a day + env. triggeer. But I have to admit that with all these arguments, by RAW, it is truly a constant effect, no matter the location of the item and its target.
I have mixed feelings towards this ruling. It is good that there are some ways to achieve constant effect, but I am not pleased with the way to achieve it. As other suggested, I would rather have an special parameter for item "Constant effect", with an appropriate costs (+5 or +10 levels) than the current situation which sounds mechanically/logically weak.
I have to say that this "truly constant effect" interpretation greatly surprised me. I would never have read it that way; like virtually every person I know, and most I see on the forums, I had assumed a twice/day "seamless" recasting (and I had thought it a very elegant solution for "simulating" constant effects). Since 90%+ of players get it "wrong", it may be worth asking David Chart for a clarification in the errata.
Before that, however, I'd like to communally brainstorm on the implications of the "cast once, lasts forever" interpretation". As was mentioned elsewhere, an important (and aesthetically not-so-nice) implication, is that such "items" really work as one-time, permanent Rituals. Unless the effect targets fhe item itself, after the first activation you can hide away, destroy, Disenchant, render for vis, or "mutate" (as per Ancient Magic's mechanica) the item without any consequence on its magic.
Another crucial can of worms is how these effects interact with PeVi. It seems that a single PeVi, non-ritual spell can effectively "destroy" the item: technically, the item remains enchanted, but its one-time effect is destroyed and can't be repeated ever. Somehow, if this interpretation were used in my sagas I have the impression that no-one would ever bother making "Constant" items.
Any other issues?
I realize that I'm comparing different "systems" of magic, when I mention Bilbo and Gollum and The One Ring. And while I agree that a device being active when worn is better modeled by D: Concentration, item Maintains Concentration, that has some problems, namely the flicker at sunrise and sunset, it's not a seamless effect that is constant, which it should be for such an item.
But an item that has a constant effect, once activated isn't maintaining concentration or really sustaining the magic in anyway, the effect is fired off and it is and ongoing unless someone uses spells to negate the effect. As you say, killing the magus doesn't negate an effect once it is ongoing. Why is it different for a device? If a device is destroyed that had a D: Sun effect, wouldn't that effect persist until sundown? If a device that had fired off a constant effect is destroyed, it should persist until it is negated. And my feeling that if it is tied to one and only one recipient this is a fair understanding. The recipient of the spell is under a constant magical effect, so accumulates warping, the device can be destroyed and the effect is ongoing, while the risk is that someone works a PeVi unravelling that destroys the effect.
Taking it a step further, using the device actually destroys the device, since it isn't needed any longer, can't be reused, and this prevents a Verditius with the appropriate virtues from reclaiming the vis and repeating this process ad nauseam.
This is quite prone to abuse. What if you up it to unlimited uses per day? It is canon that an item can maintain multiple castings of a spell just as easy as a single casting. While this is allowed under both interpretations, the "fire and forget" method would allow the effects to be geographically dissimilar form each other.
It's really not abusive. It's extremely fragile and a PeVi formulaic spell can essentially destroy a magic item with a constant effect. Instead of taking an hour or more to destroy an item, having it in your possession and in a safe place, you can destroy a season or more of work in a moment...
To be clear, are you saying this in regards to the idea that if the effect is dispelled, the item can never "re-engage" the effect, effectively rendering the item useless?
This is a concern, however as items can have spell levels higher than 50, a skilled enchanter can add enough, penetration, let's say, to render the effect un-dispellable by anything less than a ritual. Meanwhile, the item itself is not targetable because it is held in a secure location elsewhere. Paradoxically, this renders enchantments made under this interpretation either ridiculously easy or nigh impossible to dispell, depending on the skill of the enchanter, both of which sidestep the "normal" canonical means of destroying an item (i.e. address the item directly).
While I understand that one effect, never renewed seems to be canon over one effect, seamlessly renewed twice a day, it seems the long term effects have not been adequately thought out and addressed.
Yes, and the nature of Hermetic allows certain odd effects if this is truly how constant effect items work (never actually re-engaging, firing off the effect and being done).
Dispelling an effect is based on the level of the effect, which includes the R/D/T and optional parameters, such as size. Penetration actually has no bearing on it. The Casting Total is not the level of effect for Hermetic Magic (except for MuVi magics), the casting total minus the effect level determines penetration, along with some other things such as spell mastery, penetration score, arcane connections, etc. Devices don't have casting totals, so Penetration needs to be added in, and it affects the level for the purposes of determining vis consumption, just like uses per day, triggers, limited users, whether device maintains concentration and Fast Trigger are part of the device (list isn't meant to be exclusive, just trying to remember all of the effect modifications available for enchanted devices). It's been discussed before that modifications on items that raise the level of the effect to more than 30 don't cause warping. That's where I'm coming from, and I wouldn't allow devices to be that much more powerful than standard Hermetic magic. Obviously, someone under a constant effect, even if it were designed for them would get 1 warping point from the effect due to the constant nature (I'm presuming that such an item is designed for the subject, exceptions may apply). This is why I say that constant use devices are now even more fragile than regular devices, because the effect can be destroyed easily with formulaic PeVi, and the item can't actually recast the effect.
Yes, it seems to me that such an item is one and done. If the effect can never be recast on someone else, then the item is useless after it has been "used." The rules of spells cast by a magus allow the recipient to travel out of range of the caster, and have the effect ongoing, even on Concentration spells, with the exception that concentration spells that allow changes can't function until the recipient is back within range. And callen has already mentioned that if a magus is killed, after having cast a Sun duration effect that the effect is ongoing until Sunrise/Sunset. By that same logic, a Constant Effect item should keep going even when the item is destroyed (it's really out of range) because the item isn't sustaining the effect anymore (it has been cast/used), and it cannot be altered or recast on another, because the device is essentially used up.
Instead of directly addressing some of the things said above, I would like to look at what else is said in the rules. I really don't think most of this is as bad as some of what is written above. But I do think there are some things that are odd and may not be desired by all.
First, let's look at what I will call "permanent" Hermetic effects. "Permanent" doesn't mean can't be ended in some way but are not subject to usual expiration. Also, I'm just looking at Hermetic things right now. We seem to have four: the Familiar Bond (the bond itself), the Talisman Bond (the bond itself), the Longevity Ritual, and Constant devices.
Second, let's look at dispelling. First, there is disenchanting, which requires a ritual. Familiar Bonds need a ritual to break them, which seems consistent. Second, we can look at a specific note for one of the spells that uses non-ritual PeVi. Under "Wind of Mundane Silence" we see "Magical things near the area of the Wind wave slightly from the 'breeze.'" Presumably this does not end Familiar Bonds, Talisman Bonds, nor Longevity Rituals. If the other three permanent Hermetic effects don't end, why would Constant devices? But if just ending the effect means it never works again, then it would seem Constant devices can be undone in such a way. So it seems that the Constant device/effect should be treated as a "magical thing."
Third, let's look at an issue with "Aegis of the Hearth." It does not state how enchanted effects are resisted, just that they are. We don't want this to take down Familiar Bonds, Talisman Bonds, nor Longevity Rituals. So we probably don't want it to end Constant devices, either. This fits with the above treatment of the Constant device/effect as a "magical thing."
Fourth, let's look at its destruction. The same Necromancy section of Hermetic Projects that puts the lynch-pin in the interpretation of "Constant" also says that destroying the device ends these effects. So it would seem a device must still exist for a Constant effect. So there are still ways to end Constant effects: device destruction or disenchantment.
Fifth, the same Necromancy section of Hermetic Projects then has the resulting being treated as a Magical creature, so the "magical thing" interpretation seems consistent.
Thus, I believe a Constant device activates automatically when completed (as measured by sunrise/sunset at the end of the last day of enchantment) and runs eternally, sustaining an effect that can be ended by disenchanting the device or destroying the device.
I still don't like the permanent at-a-distance effect that Jonathan points out above. As I said above, I would have only allowed this for effects on the device itself. But that Necromancy section of Hermetic Projects tells me I am wrong about this. But perhaps this can be good for stories. If a permanent effect is set up like this, it will require effort (read story potential) to end the effect; it's not as simple as a quick PeVi effect. So maybe that can work out as a good thing, despite my sharing some of Jonathan's worries.
What good would that do? It would cost more Vis maybe take longer and that is it. It only effectively gets triggered one time on the sunrise/sunset when it is finished. So you give it lots of bonus uses per day that it cannot use, making the required Lab Total higher.
Canonically other effect modifications (such as penetration or concentration) modify all castings, even concurrent ones, thus, following this precedent there is no reason by RAW that Environmental Trigger cannot support multiple effects. In addition, ArM5, page 99, top left paragraph states:
"Any number of effects can be linked to a single triggering, and may depend in different ways on the result of that effect."
Thus a single device using the fire and forget interpretation can support a number of effects equal to uses/day divided by two, ignoring range, target, and duration limitations.
I would never allow this in my game and believe it is easier to house rule Hermetic Projects than most of the core book it conflicts with.
Oh, wow, how did I miss that?
That still does not follow logically for another reason: it doesn't follow the Constant effect instructions. There is a very specific formula for that, and anything other than 2/day does not match that formula. The formula doesn't say "at least" or anything else (like choosing D: Moon) to give any wiggle room. Perhaps this was quite intentional given the existing interpretation on Constant effects.
Technically, you're house-ruling ArM5 not HP, and so this may well show up elsewhere. HP just brought to light that a lot of us (myself included) were misinterpreting "a truly constant effect" in the ArM5 rules because that is the specific instance I had sent to David for the errata.