I've run through the Core book several times, and especially p.158's Intellego Vim Guidelines, trying to cover all my bases with InVi effects, but I find I still have questions. If anyone has canon references, that'd be keen, but opinions are welcome, too.
Should this be taken to mean that a spell like Sense of the Lingering Magic (Provides a hearing sense for magic) will also tell you (perhaps literally ) that there's vis around in addition to detecting magical residues?
I think it follows from the text that a variant, Sense of the Lingering Faerie, would have the same result with respect to vis. Anyone disagree?
Assuming your detection spell pierces its resistance, does the vis diffuse in a creature with Might' show the vis? Or does it not actually met the definition of "concentrated magic" until it's collected in a small physical location?
I'm concluding the use of "Hermetic" versus "of the Magical Realm" was used advisedly here, to imply that a "magic detection" spell will do only that: detect effects of the magic realm, based upon:
Do casters with "duel citizenship" (Merinita, Divine/Infernal Hermetic casters) have their effects show up in both realms? Can a Hermetic Magus casting an infernal or faerie detection spell still discern that the effect came from a Hermetic source? Or do we go with an extreme definition of "magic" that limits it to the Magical Realm?
How does one detect the realm of vis? It's a weird cross case... for most effects, spells are tailored to a realm, but vis is visible regardless of spell realm. I have several possibilities that all seem possibly "fair":
a) Follow the standard of the other InVi spells and make four spells, each of which is a "is this vis from Realm X"?
b) Make "determine the realm of vis" level 4, just like determine quantity and determine art? (But if we go that route, why not allow a "determine the realm of the effect" spell?)
c) Make the realm of vis indeterminable by current Hermetic theory.
I think that's quasi-incorrect. The Main Book's InVi guidelines imply that, at the very least, one can create different spells to detect magical and faerie effect residues. So, if you go around with magic detection sight and faerie detecting smell, you'll be able to differentiate the two. (The main book definitely also says whether or not Divine and Infernal version work is up to the SG, though I feel like there may have been some data in RoP:I that supersedes that.)
The rules for the difficulty of detecting creatures with might with InVi spells are in the guidelines after the "different spells for different realms" portion, so I've always taken it as also implied that, basically, if you cast a determine magical might spell and get nothing, and then cast a determine faerie might spell and get an answer, you know the realm of the creature. (Although I didn't state it in my original questions, really, I find this finicky. There is an implied relationship between a creature's might and the amount of vis that can be harvested from it. Therefore, assuming penetration, I feel like the nature of might-y creatures is that they should be generally detectable as Might-y, but the discernment of their amount of might, or the realm they belong to, should take more work.)
But if you get nothing from the first spell because you don't penetrate the creature's MR, how does your magus know that he needs to cast the Faerie (or Divine or Infernal perhaps) version of the spell?
If your SG runs the situation properly (meaning: Knowing the spell's level and penetration just tells you, whether your mage gets a reading of a magical presence, without mentioning whether a negative result was due to insufficient penetration or the absence of a valid target) then you know as much as your character does: "I couldn't sense any magical Might." The character is as aware of the possible reasons for this as the player. So, where is your problem?