No answer there... checked already
First I think Nimsim directed you to the strike-to-stun forum since you have a WFRP 2nd edition question. This FFG forum is dedicated to WFRP 3d edition.
I have played very little WFRP 2nd (and a lot of 3d) so my answer might not be in line with 2nd edition. But I would rule that mages and mutants does not count as magical beeings. It would be far to easy to spot a mage or mutant. Or at least the check to see a mage/mutant should be very difficult.
Now a mage casting a spell would be possible to spot with witchsight. A mutant when mutating as well, or if the mutation is "acting up" in some fashion.
Most mutations are, in my view, the effect of exposure to warpstone etc (which is magical). Unless the mutant is still carrying the source of this effect, it would in 3rd edition take a 4D check to realize the "effect of no longer present magic" at work etc. Nothing would be visible to simole magical sight.
In my house rules I also have chaos stars on magical sight cause stress etc. as looking at the "unveiled world" is not all that healthy.
Chaos Stars
Magical sight can be risky as the aethyr is a strange and at times alien environment. Each Chaos Star can trigger 1 Stress. This represents the risk of madness that continuous use magical sight would bring.
Bear in mind that this is my interpretation, so no hate please. I could be completely wrong about these things.
Magic in the Old World infuses everything it touches, but the Witch Sight only reveals its biggest gatherings. Mutants are people who have been mutated by it, but then they have been "stabilized" i.e. are almost like every other people to mage's inner sight. Chaos Spawn could be the sole exception here or a heavily mutated person. Ulfwerenars can also be classified as a "heavy-duty" mutants in this case.