someone keeps unlocking my car

[prawn]

In the last week someone has been in the car 3 times now (rifled through the dash, took all the change out of the ashtray etc. I think it's kids as nothing is gone from the boot thankfully as I have alot of handtools in there) The alarm never goes off and all the locks are up on the doors and the boot is unlocked..it's as if they have their own set of keys to my car and I'm at a loss - I've taken to emptying everything out the dash and leaving all the trays open but they still went in last night- has anyone else heard of this type of thing? Is there a way to stop this without an expensive key change? (which might not work anyway) I've got a 1999 1.8T B5