Unexplained sightings
I believe the answer is:
ufos
'unexplained sightings' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ufos that I've seen before include "Supposed spaceships" , "Potential alien craft" , "Mysterious things" , "Initially, official flying saucers" , "Unauthorised traffic from outer space?" .)