A few years ago I saw someone just simply checking for a cat head silhouette. Was enough to deter other animals and also worked to recognise if the cat had caught something.

We have a cat flap that detects the RFID chip in her shoulder. It seems to be broken since a few weeks. But our cat (and not the stupid neighbour’s cat) can open it anyways with a little prying with her claw. But luckily that doesn’t work as well when she has an animal in her mouth. So we will probably leave it broken.