Alarm goes off by itself

Ok so at 6am on this Sunday morning (when my neighbours were no doubt hoping to have a lie in) my 300 tdi's alarm decided to go off for no apparent reason and probs woke up the entire street in the process.

It rained quite heavily last night so I'm going to bet that the rain had something to do with it. I have since disconnected the interior sensors and have also sprayed WD40 on all of the door switches. I have my doubts as to whether or not those were the culprits though as the alarm still went off even when it was locked with the key. I also had a brief look for the trigger switch for the bonnet but couldn't find it.

Any recommendations? I kinda need to make sure this doesn't happen again else I will become very unpopular very quickly!

Cheers

Mine does the same thing after heavy rain, I have the usual leaky sun roofs, the water gets inside the sensors. Once dried out its not a problem. Or better still, I now just lock the vehicle using the key, this doesnt seem to activate the cabin sensors, but the alarm will sound if a door is opened etc.

Ultra-sonic movement sensors can go over sensitive, you only have one so remove it from the head lining and pull the plug if your false alarm stops then you have found the reasion.