We had a question about how astral tracking works vis-a-vis spirits. In our game last night, we were visited by a watcher spirit (which our mage assensed and got its signature). The summoner dismissed the spirit -> can we still track a link back to the summoner? What if the summoner tries to erase the signature? Can he even do that since he isn't present at the site?

Signatures remain for force x hours.
A watcher spirit is force 1 as I recall so its signature would remain for one hour, tracing a signature adds +1 to the threshold for every hour sense the link was active. However being force one the signature is gone after an hour. And I don't think he could erase the signature as he wasn't present at the the sight the spirit was dismissed but magicians aren't really my specialty.

if the spirit is no longer there, one cant use astral tracking to find the summoner.

a astral signature is a bit like a fingerprint. If one do not know the source directly, one can only say that its the same source if one encounter the same signature multiple times. Only if one encounter the source directly do one know who or what it is.

I thought that a spirit's aura would be similar enough to the conjourer's that you could search for the conjourer astrally, but the only thing I could find in the rules is that you can recognize the conjourer if you assess both him and the spirit (Assessing and Spirits, SM p.96).

So no help there either, I guess.

A spirit is ALWAYS linked to its master, and any astral link can be traced back to its master.

As long as that Spirit is around that is true. Once the Spirit is dismissed the link seizes to exist also.
In this case the mage has the signature from the Spirit and thus also of the summoner.
He cannot track the summoner but can recognize that aura if he later encounters that similar magical signature.

and then there are watchers, so inapt they won't find anyone. ever. not even their summoner. when he pokes them.

Seriously, what's it with summoning retarded wannabe spirits? Or using them for anything but astral golf.

Well, they aren't bound by the limit of only one Summoned spirit at a time..

* Speed bumps
* Guards; effective against groups without wizards. Plus, you notice if they're banished.
* Messengers
* Freaking/annoying the mundanes

Given their low drain, you could summon your maximum amount every morning, just in case. It's unlikely they'll cause you even a single stun damage.

I think Searching is one case where (imho) the GM should not limit the number of rolls made.

If you immediately began to track the astral signature while the Watcher was there, you'd be facing an Assensing + Intuition (7, 1 hour) Extended test. Each hour that passed, would increase the threshold by 1. This is the base Threshold of 5, +2 for unbound spirit, and presumes the target is not behind an Astral Barrier. (SR4A 192-3)