LET'S say you're at a park on a sunny spring day when, all of a sudden, you spot an iPhone lying next to a tree. What do you do? If you hold fast to the saying "finders keepers", you might pop out the SIM card and insert your own. But if you're a so-called "digital *Samaritan", you might log onto the Web when you return home and do a little online *sleuthing to find the iPhone's owner.