Any chance you used one of the maps lights on the rear-view mirror close to the time before your battery started going dead?

I was having the same problem with my battery running down and for the life of me couldn't find anything out of the ordinary. It just so happened though that after I'd recharged the battery once again during the diagnosis phase and was getting ready to call it a day, I'd already turned the garage lights off for the night but looked back into the garage one last time and noticed that one of the map lights was on.

During the day and with the garage lights on when I was trying to determine the problem, the interior was bright enough that I just didn't notice that little map light being on. Simple mistake, but it sure drove me nuts for a few days.