I now have the definitive answer. Cat flaps do all use exactly the same magnetic key. Therefore if those, as yet, not so clever clogs cats all realised every one with a magnet key collar could access, and thus burgle each others houses.
We know because Seren, the cat lost her magnet key and after a month or so of sellotape holding the cat flap open and feeding all the neighbourhood cats who sneaked in we found an old one. And it really works - totally different manufacturer and design and everything. So much for new technology, mind you it does beat leaving a window open and having random cats enter your house to wee in your toaster.