Besides what was previously suggested, does the car have lights under the hood. trunk, glove compartment, or ashtray? If so one or more may be staying on and draining the battery, if you do not have access to a namp meter to check the draw. I would pull the bulbs out of any of those positions and see if the problem goes away.
If your brake pedal is sagging enough to turn on you brake lights you should be able to see them on even during the day. You can adjust the brake light switch on the pedal to eliminate that issue. But you will need to address why the pedal is sagging and correct that issue also.
Also does the car have a aftermarket Radio? If so the modern radio's draw a little current even when turned off to keep all the memory information. If you have a charging issue, or even bad grounds that little current draw could be the issue.
