iPhone 4: How to find owner or Reactivate?

syntaxfree

I purchased an iPhone 4 from my brother-in-law who found it in the back of his tuk-tuk(motto-taxi) here in Cambodia. He tried to find the German owner who he believes left it behind but he was told by another traveller that the owner has left Cambodia. I tried to put the sim in another phone to find a family member to contact but the sim could not be accepted. As it stands the iPhone says to sign in with Apple ID from the original owner or re-activate. So I can use the phone for something other than a paper-weight. Thanks!