My house has two floors. These blemishes are on the first floor ceiling, along the southern wall. The floor joists run east-west (left to right in the first picture). The blemishes do not pass beyond the first floor joint. There is no indication of water damage on the second floor, as far as I can see.
However! The majority of these blemishes are directly underneath the room that has the horrible fiberglass smell from my prior posts.
I was going to drill into one to see what is going on inside there - but my stud finder indicates there is a stud in each area. I am wondering if these spots are from previous water damage - where someone perhaps put spackle over the area, sanded, and then painted it.
It does seem clear to me that someone spackled it at some point in time. I cannot explain why the stud finder things there is something there.
Perhaps I just have to cut open a hole near one of them and stick an endoscope up there.
Edit: mouse confirmed. I used a hole saw to cut a hole in the ceiling and stuck an endoscope up there.
It gets even worse: it looks like mice have access to the entire subfloor, due to a power cable (?! there's no power over there) and holes in the wood: