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!