I found an AT&T iphone 5 in perfect condition. What can I do?

I found an iphone 5 under the couch cushion at a hotel I was staying at. The phone isn't locked, so I was able to go through the owner's contacts. I tried to contact the individuals "mom" and "dad," but I was unable to reach them. In fact, the entire phone is no longer in service, so I assume the owner bought a new phone. I already have an iphone 5 on verizon, but I was wondering:
a) Is it possible to make the phone work with a verizon account (I'd give it to another family member), or
b) How would I go about selling it? I would obviously want to clear the phone completely before doing so.

Thanks for any insight you can provide. Much appreciated.

Added (1). Charles, as I said, I have left messages with my phone on the parents' cell phones, and no one has gotten back to me. I've done my part. But thanks for the advice.

If you want to return it thats the right thing to do. If it was reported lost or stolen the imei number is blacklisted and when you go to try and get it turned on they will see its blacklisted and they will not give it back to you. Or if you wanted to be evil unlock the phone and sell it. Also they can track the phone when ever it has data.Wi-Fi on. And they can use the police to come get it claiming you stole it. If you want to keep it. Then shut off all data on the phone Wi-Fi included and hook up to itunes and click restore not upgrade restore. The owner then won't be able to track it. But after all it cost alot of money and the right thing to do would be return it. If you lost it then you would like it back

It will be blacklisted once reported lost or stolen, making it useless as a phone.
you can't make it work on verizon.
email me if you need a verizon iphone 5, i have a few for sale or trade.