There is an alarm sounding in the cabin. It is coming from the passenger side rear speaker. It is not a radio fault as the radio functions normally. It is clearly an alarm.

There is no combination of top up down, doors opened/closed, hood and trunk opened and closed, towing motion detector turned off and so on, which will silence it. On the way to work today it decided to stop for 10 minutes then restarted. It continues with the car running or shut off.

I've disconnected the battery for an hour and reconnected it. The alarm starts right up again.

Any ideas?