Is it your phone? If not your phone, you can press the home button for Siri and ask, "Siri, Who do you belong to?", and a contact profile may appear. You can then contact that person to return the phone or find out the passcode. That is if it wasn't wiped.
Not only that you can not by pass it, but if find my iPhone is enable, you can not even factory restore it without the owner's iTunes account password. And unless you have register it on Apple website with your name and iTunes ID, they won't be able to help you too. Good luck.