Coolant level warning

[BlackAudiA8]

I keep getting a warning on the dashboard telling me to stop the engine and check coolant level. The engine temp indicator still stays rock solid at mid point but the warning is very disconcerting. I have checked the coolant level several times after the engine is cooled down and the coolant is fine.
any ideas on what the problem could be. Maybe a faulty sensor in the coolant tank or elsewhere in the system?
Running the VAG-Com hasnt yielded anything either.
Thanks in advance.