Fbii xl-2 alarm randomly goes off?

I have an older fbii xl-2 home alarm system. I inherited the system when I moved into the home last year. Last night the system just started going off for no reason and I’m not sure why. The only way to stop it was to unplug the backup battery and disconnect the ac power wire. We don’t arm the system, just use it for the beeps when doors open with young kids. Any thoughts would be appreciated.