I spent an inordinate amount of time trying to track down a low-battery-altering smoke detector one night in my last apartment. It wasn't the device in my living room, but I could NOT figure out where the sound was even coming from. I tried to just go to sleep and ignore it but when I laid down, then it sounded like the BEEP was inside the wall right next to my head.

I can't recall now if it turned out to be one of the two alarms in the common stairwell (which needed new batteries at various times) OR the surprise! second alarm in MY apartment (mounted high on the wall in my little entry hallway, next to the doorbell box and some other mysterious plastic box).