Hi Steve,
I'm not sure about the A6 but this is what I've found with the A3's
if you hear a bleep from the boot when the car has been parked for a week at first then as frequent as over night just after you turn on the ignition it is possible the batteries are failing if you have VCDS scan central locking module 35 (A3 8L) check for errors and you will see a error for the batteries sorry I can not remember what the code is.
The problem is on the A3 they leak on to the sounder PCB if left to long, I'm sure they are only designed to last 10 years before becoming a problem and any second hand sounder unit will have the same problem.