spy (noun) spy: person who secretly finds out about another's business
Sinonime: agent,
detective, double agent,
emissary, espionage agent, foreign agent, informer, inside agent, intelligencer, investigator, lookout, mole,
observer,
operative, patrol, picket,
plant*,
scout, secret agent, secret service, sleeper,
sleuth, snoop, spook, spotter, undercover agent, watcher