In my opinion, you know what's the right thing to do. By you posting your concerns here you showed me that you were struggling with a tad of guilt. You will feel better knowing that you at least tried to find the ower. Most likely the owner won't show up, but if they do then think how happy you will have made them. Think how you'd feel if he were yours and no one made that effort. If the pup is meant to be with you then the owner won't be found. You can live with yourself by at least trying. My mother called me yesterday and told me there were signs in her neighborhood that said someone had lost their 4 lbs Yorkie. You wouldn't happen to live in Florida?
Good luck!