Same happened to me and is still happening. I didn't realise this was happening immediately as my bedroom is at the back of the house and I park my car at the front. I would just wake up to the tyres being let down or it being covered in scratches.
In the end I left the indicators going but disconnected the alarm horn. This is behind the trim on the left hand side of the boot/hatch back compartment.
At least my neighbours topped lettintg my tyres down overnight! The indicators going off alll the time has killed 2 batteries in 3 years though.
Karl
November 2007