This also applies to any detection spell you'd cast. Sure, you could use touch or eye range to be stealthy, but this also applies to DEO.
Also, note that DEO may easily be of level < 30, thus avoiding warping, while, if you make detection hard, it may be harder to do that with your detection spell
With a detection spell, you might also miss a demon.
As for triggering an attack, all that would entail is more DEO, so the results would be minor, especially considering things like this: Multicast 3 level 20 DEO to someone with a penetration of 30, and, if it's a demon, he's toast.
Likewise, a demon detection spell would be useless with a proxy.
=> I'm absolutely not saying the DEO solution is perfect. Just arguing that, in most ways, it is already a "demon detection" spell, and a pretty efficient one to boot.
I mostly agree with your objections, but these would also apply to demon detection spells.
In fact, a really useful breakthrough for the OoH would not be the ability to detect demons, but the ability to force them to tell the truth. Problem is, that goes against their essential nature :-/
One thought missed in the earlier discussion of "if demons can't be detected, why do they behave as if they can be detected? They should just be everywhere, acting with impunity."
The inability to detect demons is a hermetic limit. It only applies to hermetic magic. Other Magic or Faerie domain magical abilities, or even hedge magic abilities might not be subject to this limit. Leaving that aside.
Sense Holiness/Unholiness and a host of Dominion-type abilities can and do detect them, either directly or indirectly. Also the world has a lot more priests than magi. This means it makes sense for demons to act cautiously. Even a covenant outside of Dominion tends to have an aegis to keep the little guys out, and significant risks of actual destruction for a more powerful demon if the Magi somehow do detect his presence (by some means other than InVi, which could include something as simple as a grog or companion or even a visitor to the covenant with one of the supernatural or dominion-type virtues).
Yes, except you miss out on one point at least i was making...
How do you know those nonhermetic abilities actually work and isnt just fake powers given by demons?
Answer is that you dont unless you metagame and "know it" based on knowing what the character sheets and rules actually says.
If there is an "infernal limit" of one sort, what´s to say it doesnt exist elsewhere as well?
Well one in-game method for determining the efficacy of a measurement technique is the same as the real world method. Repeated successful use in a variety of situations. The more successes you have the less likely they're the result of random chance or some grand conspiracy. As I've said before in this discussion while demons in Ars Magica are masters of deception I don't buy the entire host of hell cooperating seamlessly on this kind of long con. One prideful imp who's not as smart as he thinks he is could blow the whole deal for the rest of them.
Characters with faith in God should trust divine powers as a matter of course. Not merely because of their faith. Information gained from divine sources should be suffused with a holy truth other realms just can't copy. Infernal powers might try to hide this deficiency by playing off the users pride, wrath, or other sinful impulses.
For characters starting the game with detection powers both of these sceneries are entirely valid in game reasons for a character to "know" his abilities work. If you want to make it a story that that character is mistaken. Then it's up to that storyguide to provide the clues that the character is wrong.
Furthermore while coming to understand and trust abilities created in play is natural fodder for stories it's not completely wrong to gloss over those details either if those aren't the stories everyone wants to tell.
ST "Yeah you know that spell you guys developed to detect demons a year ago but weren't sure it worked."
PLAYER1 "Yeah"
ST "Well the Orders Pretty Sure it Works"
PLAYER2 "Really what brought them around"
ST "Well partly it was confirmation by trusted holy allies and companions, Partly it was the many diabolists found within the order, But mostly it was the giant army of demons declaring war on the order."
PLAYER1 "So where at war hot damn my Flambéau just tricked out his talisman."
PLAYER2 "Dibs on the DEO mastery book."
The same could also be said of Hermetic Spells. When casting a DEO (or your breakthrough-related detection spell), or any spell at all, are you sure it works, and/or ain't the result of demons?
This could be used in a kind of dark campain in which magi discover that all of their spells are actually produced by demons duping them and that they're in fact no different from mundanes, but it just ain't the standard Ars, although it could be fun to have the players discover they gave big time into Pride and Other Sins (that'd make a good saga or book name IMO)