Jack the car up "(a few degrees)" and check the fluid level without starting the car. When cold it will be very high above the reading on the dipstick about 80 mm.
Pull the carpet up from under the drivers feet in your car [here in the States it would be the passenger side] so I'm guessing your driver side, you will find the Transmission Control Model (TCM). look carefully and see if there ever has been any water in that area. Also be sure there is no oil wicked up thru the wires to the TCU.
If the mechanic erased the codes he has a code reader that may help you solve this problem. Try scanning as a 2004 Mercedes SLK 320.