we used to have the same problem mate the back garden gate would be shut on a night but open on a morning so we started shutting the big gates on the drive aswell and sure enough they were open next day... turned out it was a lad from our street climbing over the back fence on to the garage roof dropping down and going home ( shortcut from the pubs) twat soon put him right though.. not saying this is your problem but ya never know..

CheeRS
Craig