Obviously my poor ceiling, at some point in the past, fell victim to the wily ceiling vampire, most dangerous of the architectural undead, which shuffles in, deep in the night, to drink the plaster of the living, before escaping into the twilight with a whuffling of great pink insulation wings.
No more shall my once happy-go-lucky ceiling romp carelessly atop its walls! No, it's doomed to crouch sullenly in the corner of the apartment, listening fearfully for that familiar lathe-and-drywall creak on the stairs, wondering if tonight the ceiling vampire comes back to...finish the job.
Or possibly there was a light fixture there at some point in the past, and those are the holes where the screws were removed, but c'mon! Where's the fun of that?