Audi 2001

This is unfortunately common. Charge the battery, put a voltage tester ($10 most places) to the battery and have someone start pulling fuses. You are looking for a minor change. My friend with an '02 Allroad found his radio to be the issue. You could pull the fuse from yours to see whether the charge holds then. To kill the battery overnight, the parasitic draw would be pretty big. Are you sure doors are latched and latches work so no puddle lights on floor or overhead? Just to be certain, while testing the battery when running it should be well above 12v (~14v or so) which tells you the alternator is sending current TO the battery.