I have had this a couple of times over the year I have had my car too. The last time it was when opening the door after a longish sleep (Couple of days of no activity). No science or technical knowledge to my theory, but I put it down to the car waking up and it being a bit slow on the boot up process and possible not got to the point of turning off the alarm by the time I had opened the door, so it had triggered the alarm. There may be some code if the alarms is triggered check to see if the locks have been disarmed which it did and so stopped the alarm after one pip.