Alarm went off while disarmed

Recently had painters come and while they were there the ring alarm went off. The system was disarmed and on top of that, the key pad and base station station were unplugged . Any reason why this could happen or what could cause this? Even while it was unplugged the system is on back up ( unless the back up lost one’s back up battery) allowing the chimes to come thru whole doors were opened. Any info would help. Thanks!