Brian,
I am sorry to hear of this. Working with the police is always the way to go, and you can maybe call and ask them when it will be returned. Unless they need it for trial evidence...then you might be out of luck, but give them a call and ask. I also am with DylanGypsy wondering the part the landlord plays in this. If he did not have the check, how did it get to the 3rd party? Can you call the city attorney for some free advice on this issue of what to do for rent. I am sure as far as you see it, it was paid. But I suppose it is like taking a test in school...meaning unless it is in the grade book, it is considered undone. Get a receipt from now on, too, Brian. That would be your proof of payment (dated, signed, and what for)
Kindest Regards,
Frinkie

Ran's answer has left me laughing out loud here. He must be a police officer himself. Reminded me of my youngest son's wedding. He is a corrections officer and the men's side of the wedding party looked like a line-up of big, heavy-duty pro wrestlers. They all worked at the maximum security prison together. I leaned over to the hubby and said, "I guess we now have the answer to the question 'Where's the beef?" tee hee.