Certainly it is a deviation from the normal behaviour.
Here I disagree: In the expression "Off we go" the idea of a start is clearly reflected. Also in talking about an explosive charge, going off describes the fulfillment of its normal purpose

I suppose that's also true. But I meant that most of the time, the alarm was NOT ringing...Implicitly defining "normal behaviour" == "what the thing spends most of the time doing" (as opposed to what it was designed to do).

Therefore I both agree and disagree with you, wseiber.