Accenta + Goes off in the middle of the night!?

Hello, i had an Accenta + Fitted to my garage. Not the house the garage only as i have an expensive motorcross bike and they have a habit of gettin stolen. anyway it has one P.I.R and two sensors on each of the doors. i put the alarm on and sometimes it goes off without any reason, its not showing any zones just the tamper light is flashing. it dosnt matter if it windy or not. sometimes its o k for weeks but this week its gone off twice in the last three days at 2am and 2.30 am ! i also have an additional blaster which is about 130 DB so it wakes up everyone in our street ! Can anyone help? Thanks keith. oh and a couple of times the keypad goes off but not when the alarm is set?? but still the tamper light flashing. cheers