Now here's a thought about a project for players to take on; if the Order of Hermes were to (grudgingly) accept some Divine aid and found out that Infernal vis had three tiers of corruption, why wouldn't they work around that?
Say, the Kararites in Ex Miscellanea find out about it, and they work together to enchant a literal bronze box to cast Cleanse The Verminous Vis three times in a row on anything inside it. If it's strong enough and cast by a Cautious Sorcerer, why NOT just spam it on every pawn of vis that passes through the covenant? In fact, once the Redcaps find out about it- and they WILL- they'll almost certainly commission a copy of the box for Harco. Bring along that one token lunatic who was born with Sense Good and Evil, and suddenly the Order can put a seal of approval on every pawn that has been officially processed. Now, such processing would almost certainly have multiple points of failure. The bronze box could become Warped or be tampered with at some point, or even be swapped out with a fake. Whatever Evil detecting inspector is watching over the process could be killed, impersonated or charmed. Heck, getting the order of Hermes to trust Holy Magic is a quest in itself- nobody likes being told they're doin' it wrong. But even with all these flaws, a little bit of literal praying is a better method of checking for Infernal traps than shrugs and metaphorical prayers.
Demons are written as he medieval mind thought. Being Angels they have the ability to see things as they are, unclouded by intellect, without need for reason. They just see truth. They are also infinite liers. Doesn't matter how dumb, they lie infinity.
This is why they are no fun. They all require Divine to save the plot. Deus ex Machina, in the literal and figurative senses.
Nothing one will just "let the Divine" anything. Being completely undetectable is subjectivity no different from nonexistent. It is impossible to tell. You have spells that say they do things. The spells only seem to do anything when demons want them to.
So you have a political enemy that conveniently is the only one that can detect the "demons". They say your spirit totem is "pagan" and then you di. Fun game.
This is specifically not true in Ars 5. RoP: Infernal p 31 says
Even considering that they were once angels, their knowledge from that experience is limited to the portion of knowledge which was revealed to them before their fall, and from that knowledge only the subset of material that they still recall. It further seems unquestionable that a great proportion of what they recall from what was revealed to them before the fall is either no longer true or no longer relevant thousands of years later. All in all, my inclination is to consider the revealed knowledge possessed by demons as, with some exceptions, irrelevant and trivial.
Also, look at the checklist for roleplaying demons on page 46 of the same book. Demons have real difficulties in acting rationally at all. They're spending confidence points like crazy just to avoid abject stupidity.
aunguard10, I don't understand. From how you describe your experiences with Ars Magica, it seems like you have a very hostile game master who is trying to screw over their players. Have you played in a campaign where the GM tried to remove player agency through creative rules interpretations? If so, consider simply leaving that game instead of continuing to let it hurt you. Infernal enemies are mostly part of a splatbook expansion, and if you don't want them in your game, details on them are only a small fraction of the core book. If you consider fighting demons to be un-fun, you can very easily start a campaign without them or join one without them as a focus. You are ultimately the final arbiter of what gets to be in your tabletop game; I have seen similar house rules concerning the Faerie realm, stating that the post-modern explanation for faerie behavior should be ignored so their motivations could remain mysterious.
BACK ON TOPIC, who here has messed around with the mechanics for experimenting with Faerie vis in RoP: Faerie? How easy is it to reduce the chance of catastrophic failure? SHOULD it be easy to make it safe, or does the wild nature of the magic make it more interesting?
It seems like the majority of people agree that realm-of-vis can be sensed by magic; The possible exception being infernal vis that is actively hidden by a demon.