@cercerilla could be that the colder temp in homes at night makes the nearly dead battery less able to supply current and makes the measured voltage cross the threshold into beeping territory.
@sharp that does make sense, we keep our house quite a bit colder overnight.
@cercerilla if the detector is not also AC powered and not connected to a network of other detectors you could pull out the battery and hit the test button to drain it of it's ability to make beeps.