GPS gizmos, including one you can put on your cat so you know where it goes
I'm not all the sure I want to know where the cat goes, though I suppose it could be a form of geo-caching where you could go back later and dig up the 'treasure'....
The important thing is not so much where the cat goes, but how much wildlife it happens to kill indiscriminately, and how many opportunities it has to pass on its murderous genes.