I would suspect that your alarm microswitch in the lock mechanism is going bad. Chances are that you are arming the system when you lock the car with the drivers door and then the faulty switch on the passenger's side is not disabling the system. When you open the door, the car thinks it is still armed and sounds the alarm. I have a similar issue on my 300D and I've disconnected the switches until I replace my faulty driver's side microswitch.
HTH
