I'm dealing with the exact situation as Rod. Took down detector and chirping is coming from the ceiling. Not sure how to go about looking for another lost detector though. My basement was finished 4 years ago and I just cannot see the electrician leaving a smoke detector in the ceiling. I mean, the ceiling was put up prior to his work and he would have just been drilling holes for fixtures and detectors. Not sure how to follow the sound, but, it chirps every 20-25 secs, all 5 detectors in the house just had battery changes.
the barracks I stay in when on duty is two story . .and originally someone went ballistic..there are smoke detectors spaced . .WAY WAY too close together . .they are ALL 9volt battery powered and they NEVER replace the dang batts when they die . .so "chirping " is part of life . . .every single stickin' hour you are in there trying to sleep