Audi Repair Questions? Ask a Mechanic for Answers ASAP
This is called a parasitic draw, and there is no "common cause", it could be anywhere. The only way to find it would be to set up the battery with an amperage meter, latch all the door latches and hood latch (but leave the doors and hood open so you can access the interior) and then lock the car up, so it will "go to sleep". At rest, with all the control modules at rest, you should not have more than .03a draw. If you have more, then you need to pull one fuse at a time to figure out which circuit has the draw. Just be aware that sometimes pulling a fuse will cause the system to wake up, in that case you need to let it go back to sleep again. One fuse at a time, put them back in before you move to the next one.
If you get reminders from JustAnswer, know that they are automated messages and not directly from me- I won't harass you because I know some of these tests can take a while, especially if working on cars is not your primary occupation (and you may have work and family to work around) Just get back to me when you can.