Magus A wants to cast a spell on Magus B, when Magus B suddenly vanishes. Magus A guesses that Magus B, an Imaginem specialist, must have cast on himself an invisibility spell. Question : can Magus A cast a PeCo spell on Magus B using Voice Range ?
Same case, but Magus A wants to cast a PeVi on the invisibility spell. Can he do that using a Voice Range, although he does not "perceive" as such the invisibility spell (but nevertheless "sees", or at least can ascertain with its eyes, the result of it) ?
Magus A wants to cast a spell on Magus B, when Magus B makes a wall appear out of thin air between them. Knowing that the voice of Magus A can still be heard on the other side of the wall, can Magus A cast a PeCo spell on Magus B who - as far as Magus A can know - hasn't had the time to move away from where he stood before ?
I guess the starting point to these questions is the Limit of Arcane Connections (ArM5, p. 80), which states : "Hermetic magic cannot affect an unsensed target without an Arcane Connection. This is widely believed to be a flaw in Hermetic theory, as Intellego magic is much less tightly bound by this limit than other kinds. Intellego can determine, for example, there are any people behind a wall the magus can see, but Perdo magic cannot affect those people until the magis is aware of them" (emphasis added).
This passage seems very clear at first reading, but then many questions come to my mind : what does "unsensed" exactly mean? If it must be as synonymous of ""unaware", then indeed the ability to sense would not be restricted to physical senses : I could also use magical senses, as the wall example suggests. But then, does guessing or, even better, knowing that the target is there (it is after all only a question of degree) suffice?
I'd be glad to hear your thoughts about this, even if I can't perceive them yet!
The answer to all of them would be... He can not target the magus directly because he can not sense him. He could target the room. Unsensed is not detecting them via sight, hearing smell, touch or taste (ick). Ways around it might be InAu25 Eyes of the bat which would perceive invisible things. Guessing they are there could not work because it would break the law. What you are looking for is the Gruagach voice range from Hedge Magic. They can target anyone that hears their voice.
No, he cannot cast a PeCo (or any directly targeted spell) at the invisible magus, because he cannot percieve him. He needs a way to percieve his target, whether by his shadow, by touch, or by some other sense. Hearing may be enough, or not, depending on your troupe and storyguide. Personaly, I'd say that you need to be able to sense him well enough that you could hit him with your hand on the first try, if you were standing close enough to do so.
You can use R:Voice, but only if you use T:Room, or if you can somehow sense the spell (through InVi, for example). Same thing as above -- you need to sense your target. Only alternative is to use area-effect magic.