Is this a recent problem? IF so, maybe your alternator is about to go out
and have not been charging the battery fully. To check, hook up a
voltmeter on the battery treminal with the engine idling. You should get a
steady 13.5 to 14 volts.

Second, don't forget to check the trunk, hood, and glove compartment
lamps. The cut off switches may malfunction, thus these lights stay on and
draw small but continuous power.

Thirdly, how old is the battery? if 3 years or more, it may have an
internal resistance built-up that causes the power to leak internally. I
buy the cheapie Walmart kind and not pay for the 5-6 years kind. To me,
those are like paying for battery insurance/replacement ahead of time. i'd
rather just buy a new cheap one every 3 year.