In past saga, we always assumed that many demons could see through illusions but after thinking a bit more about species, I tought, how can masters of deception be better a perceiving something that is not?
By destroying the species to make you invisible, why would the demon be more able to perceive the magus than say an animal or very skilled tracker?
Is that a flaw in hermetic theory? Do faeries perceive some species that are not being destroyed under PeIm? I imagine that demons can have some sort of InVi that would detect magic of say the parma or even simply the soul of the magus if he could penetrate the parma ... but is it safe to say that he cannot perceive the destroyed species of the PeIm spell which makes it that much more difficult to identify the magus for both demon & fae?
Demons (and any supernatural creature) can do pretty much anything you need them to. There should not, imo, be one blanket rule that covers all demons. So, in your saga, "most" or "few" demons could easily see thru illusions, or have a better or worse chance to. They are simply not all cut from the same cloth.
Uh? I thought Faerie Sight and Second Sight were quite different and virtually non-overlapping. Faerie sight in a nutshell allows the user to recognize, interpret and see through glamour, and to recognize arcane connections. That's all.
To quote Realms of Power: The Divine, 'Angels know things directly through their forms rather than relying on species or sensation.'
Presumably, while demons are corrupted and inferior in many ways, at least some of them do something similar. You can't hide by destroying your species if the thing looking for you isn't using species to find you.
If demons are interested in "the soul", then presumably some may have a way to detect such despite anything else. "The soul" does not conform to any Hermetic Form, and in itself represents one of the limits of Hermetic magic, so how exactly a demon's senses work (or do not) may be a matter more of conjecture (or at best sketchy empirical observation) than any demonstrable process. That is - short of divine insight (which could always be a demon's deception), how could any mortal really tell?
Your blood is an arcane connection to yourself; as is your clothing.
Since they can see arcane connection, they can see that mystical invisible link. If they can see a link on you, it doesn't matter if you are visible or not. Your connection to your life is still there and you can't get rid of that. Even your body will remain a link to your spirit for them, and so, will be able to see you after your dead. Unless the Divine is involved, of course.
Please note the word "recognize" - this does not grant them any special power to notice ACs, but simply to realise that something observed happens to be an AC.
That said, in earlier editions of Ars Magica, faeries (and I think demons?) explicitly were able to (sometimes) see through illusions - of which invisibility was mentioned as a sub-class.
This is not currently true in ArM5 - though there are plenty of ways for either fae or demons to have second sight or it's equivalent.
This situation is not unlike how old-timers may remember that demons are immune to fire - or rather were, since they are explicitly not in the 5th edition.
For faeries its simple: if this particular faerie's story requires it to be able to see the invsible, it has second sight, probably around 11.
If it's particular story requires it not to, it won't.