spy

(military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors

a secret watcher; someone who secretly watches other people; "my spies tell me that you had a good time last night"

catch sight of

watch, observe, or inquire secretly

catch sight of; to perceive with the eyes; "he caught sight of the king's men coming over the ridge"

secretly collect sensitive or classified information; engage in espionage; "spy for the Russians"