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1. Keeping a secret or furtive watch
    
2. The act of keeping a secret watch for intelligence purposes
   - undercover work
    
3. The act of detecting something; catching sight of something
   - detection, catching, espial, spotting

1. Catch sight of
   - descry, spot, espy
    
2. Watch, observe, or inquire secretly
   - stag, snoop, sleuth
    
3. Catch sight of; to perceive with the eyes
   - sight
    
4. Secretly collect sensitive or classified information; engage in espionage
   "spy for the Russians"

Derived forms: spyings

Type of: comprehend, discovery, enquire [Brit, Cdn], find, inquire, intelligence, intelligence activity, intelligence operation, investigate, monitor, perceive, supervise, uncovering, vigil, watch

Encyclopedia: Spy Spy, Belgium