I have the following situation. My cat goes outside every day, the problem is I have no way of telling when he is back around the house unless I go outside myself. I need some small wireless device I can attach to his collar that would alert me when he is in the range of my WiFi. GPS locators are just too big for a cat and movement detectors are not really working. I need something that's at max 1cm in size and simple. A wireless mini camera could do the trick, it would start streaming when in the vicinity of my WiFi, but the problem is the privacy laws in my country. If the cat entered someone's property with the camera, I could be sued. So I need an alternative, some simple small signal emitting device or something that can be picked up by my WiFi so that I know he's around, nothing more.
I should mention that I have no idea how these stuff actually work, so something I've said here could sound dumb I guess.