Garage can't reset the service indicator?

Hi all.
I just had my 8 year old A6 serviced at a garage, and while their
tool normally manages to reset Audi service indicators, it wasn't
able to do mine. They tried another garage, and they couldn't do
it either. They have said to me it won't affect the car, it'll
just annoy me, and it perhaps points to some kind of problem
somewhere in the dashboard? Just wandered if anyone has any clue
really.
Many thanks for any thoughts, even guesses are welcome.
Nev