My Ex did this to me a few months ago 2 days in a row and then didn't do it again until now...
Battery is almost new and alternator charges fine but every once in a while when it sits it drains the battery. I can hear an audible click and rumble coming from the front end behind the drivers side headlight under the air intake.
I've got a boost coming but I'd like to be able to start her up again to go to work later.
Well the battery "was" almost new. Not anymore. Yes, that sucks. In fact the battery "may" be junk now. Discharging an auto start battery, even a couple times, kills them quick.
What you want to do is perform a parasitic load test and find out what's drawing down excessive. Power door locks, rear wipers, aftermarket sound systems are the usual suspects. You'll need a digital voltmeter that measures milliamps. Keep the battery on a charger till you get that sorted out (assuming it passes a load test and isn't shorted out internally, etc) It's important to remedy this because a bad battery can damage a good alternator.. Which will eventually damage a good battery, which.. Well you get the idea.