Synonyms and related words :
Argus, CIA man, Charley, airplane spotter, beagle, cloak-and-dagger operative, counterintelligence agent, counterspy, dick, double agent, espionage agent, eye, fire patrolman, fire warden, fireguard, flatfoot, forward observer, gumshoe, gumshoe man, hawkshaw, inside man, intelligence agent, lookout, lookout man, military-intelligence man, naval-intelligence man, night watchman, operative, patrol, patroller, patrolman, picket, private eye, reconnoiterer, roundsman, scout, secret agent, sentinel, sentry, skip tracer, sleuthhound, spy, spy-catcher, tec, undercover man, vedette, watch, watcher, watchkeeper, watchman