If you can access the contact list in the phone, it's pretty standard recommended practice that everyone should record a number to be called in case of accident or emergency. A call to that number most likely would get someone who would be able to pass a message to the owner. Various "identifiers" are used.

Also, any call using that phone to someone on the owners list who has caller ID would allow them to identify "who called." They could give the owner your number.

(Note that unless you found the charger with the phone, the batteries may die in a couple of days even without using it. Chargers may be "model specific" for many phones.)

John