A money order is kinda like cash. I think legally you are required to turn it in to the police. Also they are traceable so if the person reports it lost it may not have any value.
Money orders are usually paid in cash, no I.D. Required.
In this instance the receipt is still attached so it will be very difficult to say who it belongs to.
If it’s originated at USPS I think there is a notation on there somewhere.
I occasionally have to send money orders as I haven’t a clue where my checkbook is. Everything gets paid online ; mostly.