The rules of ASig's can be found on page 182-183 of the SR4 core book.
Astral signature explanation:
1. "Magical skills and abilities produce an astral signature on anything affected by them,..."
2. "An astral signature is the magical “fingerprint” of the Awakened
creature who created it."
3. "Foci and other magical items (like magical lodges) always contain the astral
signature of their owner (or owners)."
Astral signature rules:
1. The ASig lasts a number of hours equal to the Force of the magical effect.
2. A mage using Astral Perception can wipe out the ASig by spending complex actions equal to the Force of the magical effect.
Now, this brings up a few questions that need clarified. If I have this right, any magical skill or ability I use creates an ASig. As you can see from 1. of the explanation it specifically says magical skills and abilities produce ASig's. In number 3. of the explanation it says Foci and other magical items have ASig. So it looks like there is 2 different types of ASigs. ASigs that are "produced" and therefore leave a trace, and ASigs that are attached Foci and other magical items.
What about sustained spells? My only guess is that an ASig is left behind (traceable) once the spell is first cast. Meaning I am not constantly leaving behind my ASig by sustaining the spell.
What about foci and other magical items? If I am correct about the two distinctions between ASigs, then that would mean they don't leave behind an ASig. One thing I don't get is that if I activate a foci, shouldn't it create an ASig equal to the force of the foci. However on page 191 of the foci activation rules it is surprisingly not mentioned.
Lastly, how are ASigs erased? Say I am in a room where I just got done casting a lot of spells and used vast amounts of magical skills and abilities. Do I have to spend a gazillion complex actions to wipe them all out, or do I just spend complex actions equal to the highest force ASig and it wipes all of them out?
So to rap it up. I would like to know what actions leaves behind ASigs, and how do I remove them.