1. Not observed
   - unseen 
2. Not observed
   "managed to slip away unobserved"

See also: undetected, unnoticed

• fly in the teeth of
• fly off the handle
• fly on
• fly on the wall
• fly open
• fly orchid
• fly poison
• fly rod
• fly sheet
• fly tent
• fly the coop
• fly tipper
• fly-by
• fly-by-night
• fly-fishing
• fly-half
• fly-tip
• flyaway
• flyblown
• flybridge
• flycatcher
• flycatching warbler
• flyer
• flyfish
• flying