My mk5 is now 8 months old and three times over that period I've had the seatbelt warning message come up (along with irritating chime) while driving along with the belt fastened. If I stop the car, turn off the engine and then re-start everything goes back to normal.
As this only happens very occasionally, I'm going to wait until the first service to raise it with the dealer, but interested to hear if anyone else has had the same issue.