One more, check the time that it goes off. This used to be an old trick that I fell foul of, the alarm goes off every night until you leave it off and then the next day, there is a hole where your window used to be or worse, a gap where your car was. It's amazing how it never goes off during the day isn't it? Ask your neighbours if it ever does as you aren't there right now. If it is being set off by someone, just wait for them around the right time.
My neighbour has a new Fiesta with a faulty alarm and it goes off at all times of the day or night, well it did until it was repaired under warranty a few weeks back.
Also, if you have an iCarsoft scanner, I am pretty sure mine used to to be able to get the trigger information from my CLK and it was the driver's door switch when I had this issue but a small spider in the overhead sensor can trigger them too so it's worth trying disabling the interior sensors one night and tilt sensor the next to try and rule it out.
Good luck, this can be so frustrating to resolve.