The town home next to ours was abandoned and had the smoke alarm beep every 60s or so. We tried calling the city and they were no help so we broke in and handled it ourselves.

It was a weird system where the smoke detectors were hooked into the security system which was hooked into one of those soda can sized 9v (12v?) batteries in the basement. I guess that battery was dying. Disconnected that and the beeping finally stopped.