spotter


spotter (http://definr.com/spotter)

     n 1: a worker employed at a dry-cleaning establishment to remove
          spots
     2: a worker employed to apply spots (as markers or identifiers)
     3: a person employed to watch for something to happen [syn: lookout,
         lookout man, sentinel, sentry, watch, scout, picket]
     4: someone who is the first to observe something [syn: finder,
         discoverer]