Mark the envelope RTS and drop it in a mailbox. Let the post office open it and determine the sender which they are allowed to do if there is no return address on the envelope. The sender has to pay for the postage then. Which is an incentive to address items correctly. Does sound like it might be religious tracts or similar as per SituationCake below. Or you could open it “in error” and reseal it up again if it turns out to be genuine.
E: for grammar.
Mark the envelope RTS and drop it in a mailbox. Let the post office open it and determine the sender which they are allowed to do if there is no return address on the envelope. The sender has to pay for the postage then. Which is an incentive to address items correctly. Does sound like it might be religious tracts or similar as per SituationCake below. Or you could open it “in error” and reseal it up again if it turns out to be genuine.
E: for grammar.
It was hand delivered, not sent via post. There is no delivery address or return address.
You can likely legally open it then as its not official mail.