Synonyms for Spy (same or very similar meaning)
WordNet sense 1 ((military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors):
undercover agent
WordNet sense 2 (catch sight of):
descry, espy, spot
WordNet sense 3 (watch, observe, or inquire secretly):
sleuth, snoop, stag
From the ODE community, based on WordNet
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