Congrats If you sincerely want to return the funds you can just resend funds back to the address from where it came from. It is really hard to find out to whom the coins actually belong to.

If you were previously used that address to receive payments then someone from your customer might have sent by mistake. If this is one of the possibility then check the history did you previous receive from that address. Probably the customer might have saved your address before and by mistake selected the address now.

For fun you can do this do not disclose the hash or the amount you received and just create thread that one who comes up with the correct hash and the amount can recover there funds. I am saying this because if someone lost it he will some on forums and will ask for solutions thus, there is possibility that he can see the thread.