I have probably overlooked it somehow, but is it described in canon anywhere how a skilled Hermetic investigator determines the casting wizard's sigil, without witnessing the spell as it is cast?
For example say you are trying to determine which magus cast a Pilum of Fire earlier in the morning.
There are several innocence claiming magi close enough to have done it in the time frame that you know of, whose casting sigils are:
a distant wolf's howl
a gentle uplifting sensation
the scent of orange blossoms
the maga's hair stands on end and sparks
the spells always contain whiteness
With the possibility it was another magus you aren't aware of.
Could the Hermetic investigator's own sigil disguise what he is looking for?
There is the spell InVi 30 Impression of the Faded Sigil from HoH:TL p.75, which analyzes residues of spells and effects, and also reveals the casting sigil of the responsible wizard. Spells like ArM5 p.159 Shroud Magic or p.160 Masking the Odor of Magic can still throw such a spell off, though.
Also keep in mind that when it comes to people's impression you can always duplicate another mage's sigil intentionally (especially with I'm magic), though you would still need to mask it from magical investigation.
It occurs to me that a mage's own Parma Magica could prevent them from observing another mage's casting sigil. I mean, how much Penetration does a casting sigil have?
Take the following hypothetical:
Sootius of Flambeau's casting sigil is that a smudge of soot appears upon onlookers.
Timor of Guernicus has a casting sigil that causes the sensation of a cold shiver to transverse the spine of people and animals nearby.
Vainglorius of Jerbiton's casting sigil causes every nearby eye to involuntarily look in his direction, even if they don't know he is there and/or hidden.
These 3 magi are together, with Parma up, in the middle of an empty field, and start casing spells.
Who would observe whose casting sigils?
Depends on what the sigil does.
Most examples of sigils seem to be 'side effect'. If this is a smell, sight, or sound it isn't resisted since it is simply species spreading out to the observers. If the side effect actually changes someone, then the change ought to be Resisted. A quick fix could be to have the Penetration of the relevant spell govern if the sigil is resisted.
When sigils aren't like this, but instead 'ways the spell manifests' - e.g. chaotic, well-ordered, gentle, natural etc - then Resistance isn't an issue.
If the sigil is a change to the caster, then for an investigator to spot this afterwards is tricky, unless the caster is still there. So I guess the only reasonable explanation would be that the sigil follows the active spell or its residue, and can be sensed if you can sense the residue. Regardless of whether "onlookers grow a mustache" actually happened. because the effect was Resisted or there was noone there.
Casting sigils are fluff, not crunch. Don't have them break the system.
I'll side with 'Sigils are fluff'. The residue of a spell carries that hint of your personal touch, which is enough for an observer to /workout/ the sigil; no need to see your eyes glowing brightly as you cast the spell, the smell of your sigil still lingers.
(It would be a horrible horrible opening to whole new catacomb of worms elsewise))