It was a private key, i found it in a forum.
Not in the darknet, in the clear Internet.
Best Regards!
Check the wallet address(es) and see if you can find something in the internet, if anyone has posted it and try to contact the owner.
Also check if someone has posted if he has lost the wallet or the private key of the address that this wallet has, I'm sure he will reward you.
If I were you I would give back to the owner (if he can prove it), just imagine if you lost 40
BTC, what a bad feeling.