When you disconnected the battery you reset the PCM and thus initiated another period for the car to find and code an error. It seems the car is turning the locking system off because it sense a problem and so you should be able to find a Diagnostic Trouble Code. Hook up a scan tool and see what the DTC is. Get the exact number, not some explanation from who ever hooked up the scan tool. Some codes are not available to the less expensive tools, so you may have to go to a garage that specializes in Mercedes to find the specific code. It is best to get a scanner for yourself if you haven't one already.

You should also check for water below the styrofoam that is under the carpet in the trunk.