Someone else received my rental check and cashed it...

[Kody Kilshaw]

Long story short the post office closed my PO Box without my knowledge and gave ownership to someone else.  They assured me any mail associated with my old PO Box number was removed and in a bin needing to be sorted.  If the mail was not priority mail then it could take up to 6 months to process and return to sender.  My tenants never send rent priority, so I let them know to cancel the checks and resend to my new PO Box number.  One of the tenants notified me that the original rent check had been cashed (they sent me screen shots of the check processed from their online bank system).  I'm thinking the new owner of my old PO Box number actually did receive one of my rent checks and then cashed it.  They did not endorse the back of the check.  My tenant, who has been with me for three years now, is going to his bank to try to find out who deposited/cashed the check.  if they cashed it then I don't think I have a shot of tracking them down, but if they deposited it then there may be some leverage.  

Has anyone run into a similar situation or have any advice?