Missing cap on ceiling sprinkler

I was in total agreement with Ecnerwal, that those bumps were to hold it in wall board. Finding this picture tells a different story: (notice the bumps and the threaded insert)

My best guess is that up in the ceiling somewhere, is the rest of the part; those bumps thread into it, or you can just push (see, 4.a). Be gentle with it, those are 50 bucks.

Again, I agree with E, don't touch it unless you own the building. Even then, you might want to call in whomever has the service contract for it.