Done some quick reading through the Forum and the books I have, and have not found a direct answer regarding the following:
Is it possible to determine (with Hermetic Magic, Labwork, or academic effort) what something has an Arcane Connection to?
Though it is not said, I would assume that a given thing may have multiple Arcane Connections. Some woudl be weakening, some would be strengthening. For example. A Lab Book taken from a lab might have an AC to the Lab itself and to the Magi who is currently spending the Season working on the lab text.
If something was found, an enchanted item, a lock of hair, or a dagger; is there a way to determine what ACs are possessed by the object?
It seems that you could trial an error it: InCo for example to see the last holder of the dagger... cast at AC range might get you something (noting the AC guidelines for expiration). But is there a general way to do this?
I do see that ACs are targets for PeVi spells, but not InVi spells... on purpose? Other guidelines out there?
I would expect that this should be doable with InVi since the arcane connection is itself a vim effect in essence. Extrapolating and inferring from the written guidelines I would put this as a level 15 effect, unless there are published guidelines somewhere I am unaware of.
The answer to this question fundamentally lies in whether you consider Arcane Connections to be two-way or not. That is, my blood is an AC to me, but am I an AC to my blood? There was some debate on this forum about this a few years ago ([url]https://forum.atlas-games.com/t/arcane-connections-two-ways/2348/1]), and I believe that it has since been established in canon that they not (I will try to find the reference for this).
If they are not two-way, then you cannot use magic to specifically find things that are ACs to you, although you can use an AC to detect/view/whatever the whole. I have always run things like this; I don't like the idea of people being able to cast Perdo spells at R:Arc to periodically destroy all ACs to themselves. It makes the player characters more paranoid about leaving behind accidental AC in hair, bloody bandages, etc.
There is one canonical example of something being a two way AC, and that's a magus's talisman. The process makes the magus an AC to the talisman, and vice versa. Like any item created, it's already an AC back to the magus, I just see the magus becoming an AC to his talisman as part of the process.
That's what the OP is asking, to my mind. Can he view the whole from the AC. Not can the whole view/find the AC.
Any magic item created by the magus, without having time or Perdo Vim magics decay the strength of the AC would be an AC to the maker of the item. The parenthetical language in the quotation seems to be unnecessary.
To me, that confirms that ACs are not in general two-way. The fact that the magus always has a link to his talisman is mentioned as a feature of attuning the object as a talisman, implying that this would not exist normally. We know that an invested item is an AC to the magus, but this allows as an extra the magus to be an AC to the item. Incidentally, familiars are also explicitly two way ACs.
Neither of these contradict the general pattern of a one-way AC since they are called out specifically.
I apologise to the OP if I misunderstood the query. Certainly you can discover the "owner" of an AC by using it in a spell. A simple CrIm spell would create an image of a person to which you have an Arcane Connection:
A Face Revealed
R: Arc, D: Diam, T: Ind
This spell creates the static image of a person to whom the caster has an Arcane Connection. The spell is typically cast upon a reflective surface such as a mirror or pool of water.
(Base 1, +4 Arc, +1 Diam)
That basically answered the my question... You can discover what the arcane connection is without knowledge of the target. It seems interesting that as written it does not have a Intellego Prerequisite, as it is providing new information. It certanly seems that the assumtion most of the text makes is that Arcane Connections are known before use.
Would it be legit (for my Saga) to say that the Above spell (or similar) would only produce an Image if the AC target was known? And that by adding an Intellego Req., you could create a spell that would show the target of an unknown AC?
Well the answer to your question needs to be made by your troupe. It's fairly reasonable. My particular view is that the spell should have an Intellego requisite, whether the target of the AC is known or unknown.
Now that we have answered your question, let me derail this with the idea that Arcane Connections should be purposefully taken, rather than just lucked into. Change ACs so that the act of taking the connection is what gives it power over the target of the AC. Magi should be incredibly paranoid, because they are constantly shedding hair. This poses something of a problem, as a loose hair is an AC for months! Just something to consider, as it makes the act of getting an AC something more of an exercise, and worth doing.
Maybe give the ACs a shorter duration for how long they can used as a tracking item, but keep their duration as a bonus to Penetration. Then the only way to keep them as a tracking item would be to fix the AC in the lab?
I'm not against the durations at all, and having their tracking vary at a rate different than their status as an AC seems really complicated.
I have no issue if a stray hair is acquired and it lasts for months. I have an issue that from a cosmology perspective no magus would ever allow such a hair to fall, the story doesn't make sense. Now, can a magus be tricked, or distracted while a hair is acquired? Yes, but it goes to an overt act on the part of the person acquiring the AC. The act of acquiring it is part of the power.
Harry Potter (movies) was interesting in this regard, in Goblet of Fire, to bring back Voldermort, the blood of the enemy forcefully taken was necessary to bring a body into existence. Later in Deathly Hallows, the hair from Bellatrix fell upon Hermoine during their fight, but it's clear that Hermoine paid attention to it, because she knew it was from Bellatrix. There was a purpose in her keeping the hair, though it fell on her accidentally.
It's a nuanced approach, and it isn't entirely consistent, but the idea that magi go around and leave pieces of themselves behind isn't exactly consistent, either. Hairs lasting as an AC for months just because it fell off a magus's head and he didn't/couldn't notice it while no one else noticed it seems silly. Then, if you couple that with the idea that magi in this sort of world would invest a fair amount of time and effort to scrub the areas they have been in for signs of their passing seems rather silly, too.
You want an AC to a magus, you have to be where he is, watching him like a hawk, and with purpose and intent acquire the AC. It is possible to acquire such an AC during the heat of battle, but it is unlikely and requires a rather high awareness roll (matches the scene in Deathly Hallows).
People in general leave detritus around all the time. Generally it gets cleaned up in most places it would be noticeable, or blends in places where it does not, and there are dozens of other people loosing hairs, etc. so unless the magus has an unusual hair color for the area (say a blond or redhead in Italy) it's going to be hard to find.
You're making my point. And given the OP's question, a spell that can identify said target of the AC makes such a happenstance even more dangerous. Collect all the hairs, and sort through them, casting the spell repeatedly. Since it lasts for months, you could do it in a season and count it as practice XP for the spell...
An easy way perhaps then is to just say an AC must be fixed in some way. Either in the lab or with a spell. What seems to be the problem is the casual ease at acquiring and using the ACs. Fixing them puts at least a bit of effort into that.
In lieu of fixing them permanently in the lab, you could use the exiting Muto(or is it Rego) Vim guidelines to add to the ACs duration enough steps until it becomes considered permanent, and then it can be used. That way casual acquisition is not really an issue any more.
Here's the thing, if this seems to become an issue in a saga, then you counter it with escalation and don't change the rules. I had this in a Star Wars game i was running where the PCs discovered how easy it was to stun the enemy and so threw away blasters and other weapons and just stunned the enemy. The next game a bunch of the survivng thugs (as well as those that saw the after math of a battle) prepared themselves with stun guns and such and the PCs got a taste of it, and decided that stuns weren't no funs.
Same thing can happen with ACs, if a PC magus starts compulsively collecting ACs of everyone around him, then start having other Magi do the same to him, or call trumped up chrges against him. Even if there's no merit it takes time away from activities to defend himself and likely gives a bad reputation. NPC magi can start collecting them with the same ease as well as employing other methods if they believe that other Magi are doing the same. That's generally how i handle what seem to be exploits in the rules. When there 's MAD (Mutually Assured Destruction) going on it tends to calm things down.
Creo Vim guidelines, in Magi of Hermes, insert on page 113. But, I don't see how this makes casual acquisition not an issue. It makes it even more of an issue, because it gives the magus hunting for a specific AC more time to do it. Collect all of the refuse in one place, and cast a T:Group version with additions for size on all of it, and sift through it for years... Sure, the time it takes to find the AC might be an impediment, but then again, it might not.
That works, so long as it becomes common knowledge that he's doing so. You premise relies on a social convention that such things simply aren't done. I find that a difficult fact to accept in the Order.