Same thing just happened to me. Got in my car after unlocking the doors and i hear a strange buzzing sound coming from the rear speakers. I haven't done anything different than any other day. I turn off the car, remove the key and it stays buzzing.
Another weird thing that i didnt notice before is the light on the transmission "P" stays on and so does the Start|Stop button for ignition. I didn't notice before but i think this is relevant to the sound, as i never realized those lights stay on after locking the car.