2003 Hyundai Matrix door locking

Doors keep locking and unlocking by themselves, and whirring noise coming from boot area. Runs battery down as this happens when driving or switched off. Have to disconnect battery every time we stop. A mechanic said it's the alternator that is not charging the battery when stationary, but hasn't mentioned the doors locking and unlocking non-stop when driving or switched off! He's had the car for 2 days now, and seems no nearer to rectifying the fault - any suggestions?