I was preparing my 530i for a 6 month nap. I locked the doors, then disconnected the battery, and the alarm went off. What powers the alarm? I had to reconnect the battery, then unlock the doors, to disable the alarm and turn it off.

(I also pumped up the tires and treated the gas)

 

the alarm probaly has a rechargable bat built in. You can pull the siren out but its built into the side of the rear bumper mounting bracket.