How do I return a lost de-activated phone to the owner?

If you want to return the phone to the owner after it has been wiped, then you will need to contact the carrier. Use a small paperclip to remove the SIM tray and access the nano SIM card. On one of the sides you should see a printed number which is the ICC-ID. Enter the number here to determine the carrier if the carrier name is not printed on the card. Call the carrier's customer support number and explain that you found a wiped phone and all you have is the ICC-ID number and the IMEI/MEID number. That should be enough for them to pull up the subscriber information if they are willing to do so. The only other information they might need in order to identify the owner is the IMSI in which case you would have to take the SIM card to a phone repair shop to read the number off of the SIM card. As for whether the carrier will help, I don't know.

Note, the IMEI/MEID is engraved on the back case, near the bottom of the phone.