Hi I had the the same issue with mine A6 intermittently on my rear door too , also got a central locking fault malfunction come up on the dash sometimes , I did some digging on the US forums and found it was a known issue with some A6's .
Mine would also show it was locked but when checking the rear door intermittently I would find it would be unlocked , Any way it went into the dealers 3 times before they changed the rear door lock ,