Accenta 8 alarm goes off when not set?

I was in the house the other day and the alarm decided to start ringing for no reason I could see, the panel where we type the code in was normal, with to extra lights. I then decided to remove the cover from the main control panel (had to type the code in the stop it beeping) and disconnect one of the battery terminals... this stopped the alarm going off. The light on the box outside is now flashing. Any ideas what's been going on..! Do I just need to replace the battery.
Thanks