Buff host with no clothing is what could alarm Sky viewer (4)
I believe the answer is:
ufos
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for ufos that I've seen before include "Strange aerial objects" , "Potential alien craft" , "Mysterious objects in sky" , "Supposed spaceships" , "Roswell sightings?" .)