FWIW, I once managed to drain the battery on a car with automatic headlights. Turns out there was a "secret handshake" to make the lights stay on forever even without the key in the ignition. I managed to trigger it one night by accident, and by morning the battery was dead. Probably not the issue in this case, but it can happen.
And just generally, any time I have to get a jump start, my re-charge trip always ends in an Advance/AutoZone parking lot, just in case it fails to start again the next time. Have them check the battery while you are there.
Also, if the battery checks out, then you may need to look at the alternator.