I had this about a year ago. Couldn't figure out what it was as the car was only 12 months old and thought the battery going flat in that time was strange. Anyway, found out that if you unlock the door, you have about 10-15 seconds to put the key in the ignition and start the car before the alarm goes off. Went to the dealer, paid for a new battery which they fitted for free. Then had to re-establish the link between the key and the car for the remote central locking. Can't remember off the top off my head but is covered in the manual.