Watch secretly
Else clues• Bug
• Follow
• Investigate, perhaps
• Keep under surveillance
• MONITOR?
• Observe covertly
• Observe furtively
• Observe secretly
• Observe surreptitiously
• Observe through a keyhole
• Secretly observe
• Secretly watch
• Snoop like Smiley
• Surveil
• Tail, maybe
• View slyly
• Watch
• Watch covertly
• Watch from hiding
• Watch from the trees, say
• Watch furtively
• Watch in secret
• Watch secretly: 2 wds.
• Watch slyly
• Watch surreptitiously
• Watch through a keyhole
• Watch through binoculars, say
• Watch, secret agent-style