I had the same problem, although mine tended to go off during the day in bright sunlight. Whilst yours might be a switch (e.g. the one under the bonnet near to the left hand hinge might be making intermittent contact, or the boot lid switch is also possible) it's more likely to be the interior motion sensor. I reckon that any air movement inside the car can set it off.
I had my sensor changed under warranty - at least, the dealer said that it had been changed - but the problem recurred a while later. My fix to disable it was to put a splodge (technical term) of BluTak onto each of the two lenses that sense interior movement. They are in the overhead light unit at the top centre of the windscreen. I've had it like that for a couple of years now and the only side effect is that when I unlock, the indicator lights usually flash 5 times instead of the normal one (or two?) to show that it has detected a fault in the alarm system.