alarm would not stop?

Usually when the power goes off even for a short time the alarm ' bleeps ' for a few seconds when the power is on again and returns to normal - the power going off does not happen very often.
However this time the power went off for a few seconds and the alarm went off and to stop it we had to disconnect it from the mains supply.
Anyone know why this would happen this time and how we can fix it.
It is a Colt, Optima XM
Thanks.