One time I let a friend adopt a feral cat I had socialized and showing it care. The cat was not microchipped and my friend lives over five miles away. He was trying to train the cat to be an indoor/outdoor cat, but the first time he let the cat outside, the cat disappeared. We searched everywhere.
About two weeks later, I opened my front door to go outside, and that darn cat was sitting there and started meowing, as if to tell me that I can not get rid of him that easy. He was filthy but looked well fed, probably went through every trashcan along the say, and maybe a couple birds and lizards for dessert. I don't know how he found his way back, it's like he had a built in GPS. My friend was out of town at the time, so he couldn't have found the cat and returned it. The cat found his way back, all on his own. It blew my mind, and still does.
I brought him inside, cleaned him up, and he hasn't wanted to go outside since. That was a few years ago.
I figured that if that cat wanted to be with me THAT much, there is no way I could bring myself to give him away, even though I already had/have two indoor cats. What I have found is that if you already have two cats, one more is not going to be any more work. They all use the same litter box and eat from the same bowl, sometimes at the same time.