Unlike your living
Aura, an active spell (or active focus) will have a tangible Astral
Form. Astral
Forms are obvious to notice and doesn't require a test. Odds that an astral observer will be interested in spending a complex action to observe you in detail increase a lot if he see that you also have active magic.
Astral Forms (such as your sustained spell) that run into other Astral Forms (such as a Ward) cause Astral Intersection which in turn might cause
Disruption (in this case ending your sustained spell).
If someone has a spell on them like invisibility or mask, is that immediately noticeable on the astral or does it need to be assensed?
First of all, their regular astral aura is not invisible at all on the astral plane. The invisibility spell only affect physical sight and only on the physical plane.
Second of all. The active spell will have an Astral
Form which will be obvious for an astral observer.
However, for an astral observer it is not obvious what spell class it is. Or what force it is cast at. To find out that he need to spend a simple action to observe in detail (astral perception which is resolved with the assensing skill).
If you are sustaining a spell, are you trailing behind a signature or is it just constantly active on you?
On you.
My reasoning for this:
If you sustain an AoE spell then that area will have an astral form representing your active spell. People walking into the area will be affected by the spell. People walking out of the area will no longer be affected by the spell. The astral form of the active spell in the area will carry your astral signature. A skilled astral investigator might assense the area to find your astral signature even hours after the spell effect end.
If you target a person with an increase reflexes spell then it will be on that person the astral form of the active spell will be. In this case the effect will not end when he leave the area where you first cast the spell. As the subject walk around the astral form of your active sustained spell will follow him around. Unlike the AoE example above the astral form of the active spell is not fixed to a location or an area. It is anchored to the astral aura of your target. And the active spell will carry your astral signature. If you sustain the spell and someone assense the target after the target left the area the magician will still sense the astral form of the still active sustained spell and if he is skilled he might also sense your astral signature on that astral form. If the subject walk through a ward then the astral form of the active spell that carry your astral signature might be disrupted (due to astral intersection). It can be used as a link to astral track the spell back to its origin. Even hours after the spell effect end.
