A Room is targeted by a 0-penetration, long-duration PeAn spell that causes great pain to cats. More specifically, it causes pain to all cats who happen to be in the Room at any time during its duration, for as long as they remain in the Room.
A magus with a strong parma enters the Room in human form, and once there shapeshifts into a cat.
Does his parma register a "ping" from the spell when he enters the room? (I'd say no).
If not, does it register a "ping" when he shapeshifts into a cat? (I'd say yes).
The magus returns to human form and leaves the Room, cloaked in illusion to look like a cat.
He is then targeted with a 0-penetration T:Ind PeAn spell that causes cats pain.
Does his parma register the spell? (I'd say no).
It's an interesting question. It is asking what is the perceptive powers of a spell?
Does the spell need to target something to establish if it has cat status, or null cat status? If so, everything crossing the ring the spell targets, as the spell must identify it's a cat before it can affect the target. In that case, there's a ping always.
If the spell doesn't target the magi, or anyone else entering the circle, how did the spell work out the magi is not a cat and thus shouldn't be targetted?
If we say spells with a restricted target grouping must target everything to work out if the target is in the relevant group, every time there is a parma "ping". I think this is the most logical option.
If we say the spell can distinguish the cattiness of a potential target without targetting, which I think doesn't work, then I agree with the "ping" status in the OP.
Does it actually matter if the spell could affect the Magus or not? A spell that targets them should cause a 'ping' even if it could not have an effect due to limitations/wrong target.
So the question becomes can does a container spell only try to affect things of its restricted class or try to affect everything within it? Without an Intellego requisite it has no way of identifying if something "is of its restricted class" and so tries to apply its effect on everything even if there would be no result.
Could the type of shapeshifting make a difference, ie Heartbeast vs spell vs supernatural ability? Does a Bjornaer magus with a sufficently high Cat Personality Trait get noticed? What about Theriomorphy? Also, what about a magus who has a cat familiar and has acquired "some minor characteristic of the other" via the bond?
I would actually agree with Troy here. The magic will Ping their Parma when it is resisted. Just like casting a Pilum of Fire on a magus immune to flame will still Ping their Parma. They also would notice when they walk into a room with a Ward Against Felines, or a room that is magically illuminated.