A good topic for the Halloween season.....
While this is something often seen in horror movies, I imagine it can and does happen in real life too. Dead insects and mice skeletons are fairly common, but what about other critters, or even human skeletons?
Personally, in all my years of working on homes and other buildings, the worst I found was the skeleton of a cat above a ceiling, in a building I was helping demolish. The building had been a used as a livestock auction barn.
I was sort of repulsed when I found it, even though all that remained was a skeleton and a lot of fur. Fortunately I was above it, pulling off roof boards, not below it. The building was being torn down by hand to salvage all the lumber, and I was hired to assist the guy who was doing the job.
I've also found other stuff in walls and other parts of buildings. The most common were tools (which I always liked to find), and things like spare plumbing or electrical parts. I've also found tin cans, a dish, glass jars, and few coins. I never found that stack of hundred dollar bills I always hoped to find.... And all the old homes had used razor blades behind the bathroom medicine cabinet, because those cabinets were made with a slot to deposit them blades into the wall. (which was a stupid idea)....