In preparation for the house appraiser, I got the smoke alarm back on the ceiling yesterday. Took me until this afternoon to find the "before" picture, though:
Might have to re-take that "after" photo... Fixed!
As best I can tell, the sellers had a (dead) carbon monoxide detector, a (way-past warranty) smoke alarm, some sort of siren and a motion detector on this ceiling when we bought the house. Now we have a Nest Protect.