Leaving it alone may be best
Finding a lost item is an intersting situation. It all depends on where you "found" it. Seeing a phone, bag, laptop, or other valuable item placed ina train station with the owner clearly absent does not necessarily constitute a lost item in Japan. Although it may be difficult to leave alone, the best thing to do in these cases, even if it is a set of keys or something, is to just leave it alone, and assume the person will come back looking for it. It may even be possible that they just left their things there momentarily, as some people tend to have trust in abundance that their belongings will not be stolen by strangers; although some people are a little too trusting, as of course there are thieves everywhere.