I have to say I have done this before today, but it was never intentional. I fed it tuna one day in the garden, and he had no collar on , then it sat on the window ledge with sad eyes, before you know it he was in my house on my bed and I was buying food for it.
He was never locked in, there was always a window open and I have never found out who the owner was, dispite asking!
However, in my defence, I was never a cat person until I met this cat, now I have two !! ekk thanks thats how all this happened!!!
Obviously you have made the person aware that it is a well cared for cat, and the situation although similar is different in that no one came round, I have not kept it from going home, and I have eventually got my own cats!