As noted above, look the car over carefully in the dark.
My mother called me a couple of times about a dead battery. I was confident the battery was good, I finally found the problem.
When she was getting out of the car, she sometimes managed to turn on the interior light when her hand accidently pressed on the switch, near the grip above the door. As she almost always drove in the daylight hours, she never noticed the light being turned on. On a couple of other occasions she managed to turn the lights on by rotating the headlight switch too far, and accidentally turning the interior lights on during the day. Again she did not notice as when she turned the car off, it was nice and sunny.
To run the battery down over night, points to a pretty good drain. At least a light left on, it should be easy to determine if that is the cause. Don't forget the light under the hood, in the glove box, and in the trunk.
Another problem may be a good battery, getting poor recharging. The battery may start the car fine in fully charged condition, but if the alternator is not putting out the proper voltage and amps, it will not only not recharge, driving the car will continue to discharge the battery. This often is not noticed, until you turn off the car, and cannot restart due to a low battery condition.
Do not overlook the battery cables. Often they look fine, but if there is a small amount of corrosion between the positive battery cables connection at the battery, that will create a poor connection for starting, as well as a poor connection for the alternator to resupply the battery with electrical power. Remove and clean to bright metal ALL battery cables and clean ALL of the ends. Be sure to also clean the battery terminals to bright metal condition, and be sure to clean, or replace, the battery terminal bolts.
Review any error codes and post here, there may be some information that could help.
What is the charging rate at idle with a well charged battery at idle? (over a couple of minutes)
What is the charging rate at idle with a well charged battery at 2500 RPM? (over a couple of minutes)
Be persistent, something will reveal itself.
-George