Ok, so running the first part of an adventure last week, the wizard used a tracking spell to track down a missing person to a rail yard that was moving human "cattle" for certain evil forces.
Thing is, when he got there the person was not among the "cattle". So now he's wondering why the tracking spell led him to where someone WAS rather than where they are.
As the GM, I have to tie up this loose end (as clearly I'd not thought it all the way through and was just running with the flow of the game).
I could have it be that the person was moved just after the wizard's arrival, but that seems a little too cheesy and simplistic an answer. So, what other things could make a tracking spell return a false positive, or in this case a somewhat "stale" positive?