how to stop our alarm going off for no reason?

Sat watching x factor and the alarm suddenly starts going off all over the house. It won't respond to the code apart from once but then started again 5 mins later. We switched the mains off and it made no difference. We ve unplugged the battery inside the box and pulled out a few fuses on the circuit board. It started going outside too. We put a few back in and it seems to have stopped for the last twenty mins except for a bit of beeping. Help what can we do to shut it up. We don't even use the alarm!