I would not take something that didn't belong to me. If there was a clearly designated authority figure then I would entrust them with the item in case somebody came back in search of it. At a school I would take it to the teacher or lost in found, in a store I would probably give it the manager, at work look for a security gaurd.
If it was just in the middle of the street and it looked important like somebody's handbag/ money/ wallet, laptop, eyeglasses, then it would probably warrant a drive to the the police station. If it seemed like something relatively minor like a magazine, candybar or ponytail holder left on a park bench, then I would probably just leave it where it was. Maybe they'll come back, and see it? If they don't though its not going to be as critical to somebody's life as their lost keys, phone, wallet, wedding ring, medication etc--THESE things have to be reunited with their owner if there's a way.