If you've ever found a random iPhone sitting around with a passcode, it doesn't seem like there's much you can do to return it. However, OS X Daily points out that you can simply ask Siri for help.
You have two ways to ask Siri for help finding the owner, ask: "Whose phone is this?" or "Who owns this phone?" Siri will then display the owner's phone number, address and email. This seems a lot easier than tracking down an IMEI number. Of course, if you're not a fan of all that information being available on your lock screen for any old identity thief to see, you might consider disabling Siri altogether (Settings > General > Siri).