[Essex Norm]

I have tried the obvious things such as checking the bonnet and all the windows and doors are fully closed. It is parked outside but fairly sheltered from the wind and there is no passing traffic. No sign of a cat or something jumping on it.
I've read somewhere that a low battery could cause it. The stop/start one was down to 12.01 volts so clearly not very happy. Does anyone know if the alarm circuit is supplied from this battery or does it come from the main start battery. ie the one under the boot floor? This one seems fine at 12.7v
Any other ideas welcome. Thanks.