Welcome back, that is a long trip! If it isn't the CO2 or smoke detector I'd go with option 3, the LPG detector. Look for it near the floor usually near the converter. If it is going off either your pod is unplugged and your battery is now low, or fumes from something are setting it off. Between the LPG detector and other loads the battery will last less than 2 weeks with everything "off", less if the battery is older. And I've had our detector go off from fumes from Lysol, from the toilet when its held open during black tank flushing, and other stuff.
When I first read the thread title I read it as BLEEPING NOISE. I thought you were going to complain about some nasty camping neighbor. 