Because demons can hide themselves perfectly from hermetic magics should they so choose, as pointed out above.
An unneccesary complication.
Why are we bothering to argue about this? Any demon stupid enough to fall for a simple thing like the one proposed by the OP is canon fodder anyway and hardly dangerous to magi. The ones I'd be worried about are the ones that can out think these details?
Targetting an unseen element via use of T: Room (or Structure etc) is canon under the current set of rules (as best I can tell).
There's no need to give demons a loop-hole, let them solve that little puzzle by themselves.
Because if you can target PeVi at unsensed demons you can target anything else at them too. How about area effect ReVi(Infernal) that forces all infernally-aligned creatures of might to jump up and down and sing "I'm a little teapot". Or MuVi(Infernal) that turns all infernal beings bright blue. I'd ask the question why we even say demons are impossible to detect with magic if we allow such trivial workarounds to detect them.
There is a line in RoP:D about angels being immune to any magic that would cause them to act in ways that are opposed to God, or something to that effect. Perhaps it would be best to treat demons this way - to make them immune to any (Hermetic) magic that will reveal them or their deceptions.
But by RAW, I don't see how they can avoid the MuVi detection spells. Their only "solace" is that Hermetic magic can't work as in "All demons within 15 feet" or somesuch, only Room; that makes targeting them at least somewhat difficult on the field. Sometimes. If a spell with non-standard parameters is allowed, this makes detecting demons even easier.
I can't see any problem with using DEO PeVi to infer the identification of demons. It's a reasonably clever trick, that's the sort of thing we should want magi player characters to be contriving. However, in some reasonably common circumstances it won't work.
If the demon's immaterial (and therefore undetected) it won't normally work, unless you have a Room version. And an immaterial demon who is outside is still safe. Likewise, an immaterial demon in a large room is also relatively safe (unless the magus has gone to the bother of inventing a formulaic spell with size modifiers to affect a large Room). And the whole strategy has great potential to annoy/anger/frighten other characters who misinterpret what the magus is doing.
And if you want to start making rules to stop it, then do you also start making rules to stop the equivalent PeVi effects being used to "identify" faeries? What about using Forcelessly Cast PeAn to infer the presence of a Bjornaer magus?
The thing is, demons are supposed to be this undetectable danger. Having them detectable with a little work-around isn't fun. I also find it unrealistic that other magi haven't come up with such basic tricks. That said, the rule changes should be at the level of "rules for demons", not in the "general rules".
I have had this come up in one of my sagas, and as alpha story guide I handled it the same way as Mark and Matt. That is, I decided that Perdo Vim doesn't work on incorporeal demons. My justification was that magic can't affect anything it can't perceive, unless the magus casting it can perceive it. Demons cannot be perceived with magic, and so Perdo Vim just doesn't work on them unless the magus can somehow recognize and specifically target them, like with Sense Unholiness or Second Sight.
However, I didn't tell the players that. How would they know unless they had experimented with demons? They set up a T:Room version of DEO in a constant device with unlimited uses per day. So the demon that was spying on them heard them bragging about how they were immune to demons now, and decided to encourage their pride. It got three of its minions to coagulate in the room where they had cast it, promising them they would not come to any harm, and watched gleefully and incorporeally as they burned up spectacularly. When the saga went on hiatus, no one had yet discovered their error, and I do recall a couple of planning sessions in that room that the demon carefully attended.
The other thing I did was that I ruled that a single casting of a Level 30 T:Room DEO would only destroy its maximum of 25 points of Might, divided evenly among all potential targets in the room. So, if there were 25 individuals present, each would only lose one point of Might each casting, and more than 25 and no demons would be affected. This is similar to the way I handled Group Perdo Corpus spells: the wounds were divided up among the targets, so that a Heavy wound would become Medium wounds for a group of five or fewer, and Light wounds for up to 25 individuals.
Anyway, the best defense against demons is the Divine. Demons are badasses; most can go incorporeal at will and many of them can possess a host and still be undetectable. But they can't withstand relics or angels or saints or exorcisms. After the Divine, the second best defense is being infernal yourself. Magi do at least have the Aegis, the Parma, and wards against demons, which work because they don't actually target the demons they protect against. I really feel sorry for faeries, though. Besides their Might, they don't have much they can do once a demon comes prowling around. Poor faeries. It's almost worth hiding in the Dominion, even with the huge penalties...
I agree wth much of your logic, but that is what gave me the opposite conclusion.
Probably because my demons like to work at arm's length anyway.
A detectable demon is either forced into the role by a mortal infernalist, or a cat's pawn of a stronger demon.
Besides, all of these effects need to penetrate, and only rarely will a penetraton multiplier be available - the Aegis looks like the much stronger choice from here.
Its no problem at all. The Limit of the Infernal only covers Intellego and certain types of compulsion, not all arts. You can detect demons by nuking them and watching them go feral (because they are without fortitude, so cannot hide their pain, and they will want to messily kill whoever it was that just hurt them). You can detect them with wards (and being without hope they will give up, and being without fortitude they will not have the patience to wait around in case someone comes out). You can detect them with Creo, by giving them Infernal Might and watching them crow about it. You can detect them by changing their spiritual nature to something visible, analagous to Vision of the Haunting Spirits. And you can detect them with compulsion - not to tell the truth or identify themselves as a demon, because that's ruled out in RoP:I, but to do something else (dancing a jig is harmless and humiliating, but any other service will do (you may also be able to run a Cretan paradox on them, relying on their non-compelability, but I haven't thought that through yet)).
Not all of these methods are foolproof. Some demons can hide their pain. Some have patience. Some may be able to produce illusions to mask their response (though this requires virtue too, I think), or to make other people appear to be demons. Not all are legal; knowing any spell to coerce a demon is highly dubious, and the Quaesitores many look unkindly even on knowing spells to make them visible (actually commanding them, even to do something harmless, and even with the best of intentions, is a march). And demons may deliberately target you or other Magi in retaliation. And there's always the risk of false positives, or of annoying non-demons. But if you're willing to take those risks, it can be done. The idea that demons are completely and utterably non-detectable by any means whatsoever is a relic of some previous edition, and its just not the case in ArM5.
And remember: It it is that easy to detect demons by Re or Mu Vi, why isn't this the standard procedure of the Order? While it stands to reason that it may be possible, there could certainly be consequences to magi trying it (Quaesitores or magi paranoid of such suspicious actions) or it could be argued that victims of such actions are mosty lesser demons, pawns of the greater demons to be sacrificed. Bu if demons are masters of deception, perhaps they ca anticipate the consequences of auch Re/MuVi and twist the results?
The role of Demons in ME is that they are a mysterious, unknown enemy. Because this is most useful for giving challenges and drama and makes for more interesting stories than merely using easy magic to circumvent their very concept.
Sure, and if you wanted to ward a room against demons, you could do that, because the Rego Vim guidelines let you protect a target you can perceive against demons-- even demons you can't perceive-- because that's part of the guideline. It doesn't actually target the demons, it targets the room and everything in it that you can perceive. But Perdo Vim destroys demons, directly targeting them, and you can't target a room full of demons if you can't perceive them. Just like you can do a T: Ind ReVi spell to ward an individual against demons you can't see, but not a T: Ind PeVi spell to destroy individual demons you can't see. That's just how wards and might-draining spells work.