For me it's been a stray cat.
My wife trapped him and another cat a few months ago, the other cat escaped but "Bob B" stayed.
I set up a "cat room" in our other house which is under renovation and go over there to read books, watch TV, and things like that. When I was there he would always hide on a high shelf in the closet but I had a breakthrough a little more than a week ago when he started coming down to eat his meals and I was able to feed him cat treats, sometimes a few inches from my fingers so he would bump into me with his nose. One time he bit me accidentally when he was trying to bite into a kibble and he immediately seemed contrite and got a lot more careful.
https://mastodon.social/@UP8/112824600179209599
I got him back from the vet Tuesday afternoon to get him neutered, vaccinated and all that, when he saw me for the first time from his shelf he was stressed and his pupils expanded to the point where I could not see his irises. The next day though he did come down to eat his dinner although I could only get him to do a short play session... I'm happy though that I didn't lose all my progress.
In a week or two I think we can get the rest of the house safe for him and let him have the run of the house. Right now he's obviously depressed and anxious from having too small of a space because every little thing like noises from the hard drive on my Xbox bothers him, so I think he'll blossom in a bigger environment. I'm not worried about a kitten bomb if he escapes now because I do want him to stick around because this changed from "a project I'm doing to learning more about cats" to feeling really attached to him.
There's also a concern about what we'll do when we rent the other house and we'll probably have to move him to our house (with two cats) or into the barn (with one cat) but I'm hoping he'll be much much easier to handle by then. (He chomped on me twice when I was packing him into a crate to see the vet, it's a good thing I was wearing heavy gloves)