Related Clues:

• Mole, e.g.
• Engage in espionage
• Cloak-and-dagger sort
• One ogling
• 007, e.g.
• Keep tabs (on)
• CIA worker
• Voyeur
• One staring
• Observe furtively
• Mole or plant
• Plant, perhaps
• Seek intelligence, in a way
• James Bond, for one
• Peeping Tom, for one