nissan primera p12 faulty alarm

These can be really annoying, sometime it is down to a tiny fly in the cabin but in your case if it is regular I think it is a voltage drop that the system is detecting. Do you think it occurs on cold nights?

You might want to get the battery checked under load before it lets you down in other ways, apart from that I am not sure what to suggest, the system can be interrogated by consult II but I'm not sure if activations are recorded

I think the ultrasonic cabin sensors can be triggered, when it has happened to mine there has been one flying about in the cabin, could be 2+2 =5

It took me about 2 years to figure out what made my alarm go off at night.
It happened only when parked in front of the garage at home. If I parked closer to the house, inside the garage, or any other place in the driveway, it never happened.
Also it happened between 11pm and 3am, on cold nights after sunny days, and only in the spring or autumn, never in summer/winter.

The solution: don't leave the key in the hall! I see two possible explanations:
1) WHere I used to leave the key, was near the fuse box for the house, and also that the key intersected a imaginary line between the fuse box and the car. I suspect that something (maybe a heater) triggered some electromagnetic fields around the fuse box that made the key send out a panic signal. (There is no button for panic on this key, but that doesn't mean the panic signal doesn't exist as a possible signal in the key.)
2) Where I used to leave the key, it was exactly on the limit to where its signal could reach the car. Maybe temperature changes (or other external circumstances) made the key go in and out of coverage so much that the alarm got confused.

Anyhow, I started to store the keys in another place, and the alarm has never gone off since.

Was the spot where it happened anywhere near the exhaust from your boiler by any chance? Did you ever notice pawprints on the roof/windscreen of the car? (Mine's been set off by a cat jumping onto it from a nearby roof)