gumshoe

a person not on the police force who investigates criminal or illicit activity or searches for missing persons the suspicious wife paid a gumshoe to follow her husband Synonyms dick, detective, hawkshaw, investigator, operative, private detective, private eye, private investigator, shamus [slang], sherlock, sleuth, sleuthhoundRelated Words shadow, tail, tracer, tracker; fed, Federal, G-man, narc (or nark) [slang], plainclothesman