The same thing happened to us last summer! My brother was taking out some garbage and found a teapot yorkie in our fenced in backyard. I could hear my brother yelling out my name to come out side with a bowl of water and food. I went outside and he was trying to figure out where he got in from. Our guess is someone same him and maybe thought he belonged to us and put him in our gated fence. He had a collar and we returned him 8 blocks away from us!
Then about a week later I was walking out the front door of the house and there was a tiny maybe 3lb yorkie sitting on my front steps.
She had a tag with a phone number and couldn't get a response so googled the phone number and was able to locate the owner 3 blocks away.
I don't know maybe they know that we are yorkie lovers or maybe they have a certain scent among them that they know that we will take care of them.
anyway I am sure that the word is out that if a dog is lost they can go to our house and we will find its home or will find them a home!