Bad unit. Is it a co alarm or the propane detector? Or a combo unit. They go bad at like 8 years. Co is incomplete combustion, so if you are not burning anything( stove, heat, water heater , car exhaust, etc) it shouldn't go off. Some need to be put in freash air to reset so to speak. So they accumulate co and when they get saturated go off, others purge and take real time mesurments. Your make and model should have instructions. Only other thing i can think of is a battery offgasing sometimes sets them off. Could it have a loose wire? And when it jiggles it goes on and off?