As I recall, in our 2007 PC we have 3 different detectors. I think they are for smoke, carbon monoxide, and propane. Are you certain all 3 of your detectors have good batteries? I think the propane unit is hard wired as it chirps when I kill power to the house. But it might have a backup battery inside, I really don't know.
Our Tripp-Lite inverter will also chirp for reasons that escape me right now.