On ours, you can cut the power supply outside to the circuit breaker box or disconnect the telephone line to it. A couple things can cause it to go off for no apparent reason. On ours, when the back-up battery was getting low, it will set it off. Whenever we have brown outs or power surges, it sets itself off. And, I had a problem with rain shorting out one of the door contacts when the weatherstripping around that door got old and damaged which also gave us false alarms.

Might have been a power surge of some kind.