Strange Locking Scenario
 

I have a 1999 SLK230. We just bought it and love it. The only issue is the remote locking of the vehicle.

When exiting the car, I press the lock button on the FOB and there's noise but nothing happens. If we open the passenger door or trunk and press the lock button on the FOB, the doors lock. If we close the door or trunk in less than 10 seconds, everything unlocks. If we wait the full 10 seconds, the doors all stay locked but the dome light doesn't go off. If I use the lock button on the dash, everthing works fine and the dome light turns off after about 7 seconds.

Any advise would be appreciated.