I personally can't think of anything that would give me more pleasure than returning a pet to a long time owner.
My mother once adopted a stray cat. A year and a half later the original owner spotted it in my mother's yard. They had pictures of the cat as a kitten, complete with its unique markings. They were so happy to have it back.
I remember years ago before I was married how I found a little baby kitten abandoned on the side of the road while working. It was snow white all over, cute as can be. I took it home and took care of it. One day, a few short weeks later, I noticed it was missing from my fenced in yard. I looked and looked for it, but never found it.
A couple or so months later, the following summer, I was mowing my grass, and I stopped to talk by the fence with the homeowner that lived behind me. While I was talking with her, her little daughter, maybe 3 years old, came running up behind her and guessed who followed her, the little kitten I had 'lost'. It was definitely 'my' kitten. It just showed up one day in their yard. Apparently it had wandered over to their house from my yard a few months previous, and the family adopted it, not knowing from where it came from.
Well, I kept my mouth shut, because as I stood there looking at the little girl playing with the kitten, I knew that this little girl and her family could give that kitten a lot more love and attention than I could, and that made me happy.