While modern car alarms are sophisticated and have a variety of sensors, they tend to be focused on detecting entry to the vehicle, attempts to start it or to tow it away, so it is not beyond the realms of possibility that someone levering the door mirrors from the body could have done so without creating enough disturbance to trigger any motion sensors. Other than preventing any physical access to the vehicle by putting it in a garage there is not a great deal that can be done to prevent this happening again, other than increase general security where the car is parked such as adding surveillance cameras or security lights.