(n.) One who spots.

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