The main point of this question is for it to hopefully be included in the FAQ, but I'd welcome other players interpretations:
The Astral Tracking description seems to imply that you can only use the astral signature of a spell to track down the caster if the spell is still active. From what I recall, an extant astral signature was all that was required previously. Additionally, the first condition in the Astral Tracking Modifiers table would seem to imply that an astral link no longer needs to be active in order to use Astral Tracking (for instance, a Force 4 spell cast at location 2 hours ago that wasn't wiped clean by the spellcaster).
Q: Can the astral signature of a previously cast spell, summoned spirit, etc. be used as the basis for an Astral Tracking attempt or must the spell/spirit/etc still be active?