Related Clues:

• Look in (on)
• Observant one
• Le Carré staple
• Watch secretly
• Finder of secrets
• Mole, maybe
• Bug someone, e.g.
• Catch a glimpse of
• Stratego piece
• Maxwell Smart, for one
• Watch covertly
• Snoop (on)
• Looker
• Microfilmer, maybe
• Sneak a peek