All spirits have a connection to their master that is visible to other Awakened beings who can perceive the Astral plane. Since Watcher spirits are created "from" the caster (and not pulled from the Metaplanes), I rule that the connection is particularly visible. Therefore, it should definitely be possible to track the Watcher spirit. Since Watcher spirits are particularly stupid, they won't be able to travel at their full "running" speed and still have a chance of finding what they're looking for. With those two factors in mind, you believe that there is a good chance that the spirit could be intercepted before completing whatever task it is trying to accomplish. You'd still need to track it using Astral Tracking, but you'd have a reduced threshold due to the strength of the bond that you'd be following.