My local garage has had my 2001 Audi TT car for some months (on and off) with the same on-going, frustrating problem.
The battery drains when left for more than 1 day at standstill. All diagnostic checks have been done but, nothing found.
They are now struggling and, advise that I take it to an Audi dealer! I am obviously concerned that they wil do no more than my current garage and, charge me double the cost - again!
Any advice would be gratefully received!
We had a similar problem on our 05 TT coupe. It turned out to be the number plate light which was staying on when everything else was off and the car locked. Not sure what the combination of causes was to make it happen but the solution seemed to be to open and then shut the tailgate hard.
Might be worth a look