Can anyone come up with any possible explanation for this situation?

I mailed out a book on July 2nd and it was marked "Received" on July 20th.  Today, two months later, that book came back to me!  The wrapping and label are still intact and there is a sticker on it, dated August 3rd (a month ago) that states "Postage Due  Return to Sender  Unclaimed  Unable to Forward."  I can't figure out how it got marked "Recived" if it never was and how there could be postage due because I mailed it from the post office and the sticker with the amount of postage they said I needed is still on it.

Anyone care to speculate?  (I did send a PM to the requestor to see if he can provide an explanation.)

Complete speculation here - If the senders account was suspended for something, the book would be marked received by the system.  As for the postage due, that would be for the book being sent back to you. 

Yes, I'm guessing the same as Jody. If PBS suspended their account--perhaps for a number of undeliverable books, or never marking them received or whatever--you don't lose the credit and the system auto-marks any pending transactions received.

Cheryl

Media mail is returned to the sender but only if they pay the return postage. 

Since the sticker says unable to forward, I would guess that the requestor moved and didn't update his address with PBS.  If you don't hear back from the requestor, I would re-post the book and move on.

I've had Media Mail returned to me before and never have been charged the return postage.

I was not charged anything--the package was simply left in my mailbox.  It was stamped Postage Due, though.  Examining all the various stamps and stickers on this package--and there are many--I also see one that indicates a notice was left on July 7th. Then there is one with a barcode on it, my zip code and LAX followed by a series of numbers.   I thought that that maybe indicated the package went through Los Angeles.  (The package was originally sent to San Francisco.)  However there is also another one with a barcode, my zip, and CBL followed by a series of numbers.  If that too is an airport code, then this book REALLY travelled, because I looked up that code and it is for an airport in Venezuela! Given that it took 2 months for the book to get back to me, I will probably never know where it has been!

It's possible that the receiver had an unexpected move and just marked it received as the right thing to do. 

I was not charged anything--the package was simply left in my mailbox.  It was stamped Postage Due, though. 

Being stamped Postage Due and left in your box is charging you. It is expected that you leave the postage due amount in your box for your carrier. Some post offices won't let postage due packages out of the main post office and require you to go to the counter and pay it to get the packages. Some will let the carriers take the packages, but often times the carriers pay the postage due for you and you are just reimbursing them back for the expense.  Media Mail is only one way postage. To be forwarded or sent back can be charged again.

It appears the requester moved and didn't change their address in PBS. So one of two scenarios happened. They figured it out and marked it received to give you the credit since you mailed the book and it not being delivered wasn't your fault. Or they had the problem with multiple books and their account got flagged and the site marked all of the books that were sent to the bad address as received. 

I post and get so many books sometimes I will just go through and mark them all received without knowing what I have and have not got. Bad on my part but it works for me.

Being stamped Postage Due and left in your box is charging you.

Yes -- check with your carrier.  S/he may have covered the postage for you with the expectation that you'd be good for it.

Check the account of the person you had sent it to. If the account has been suspended, I think it will tell you.

Update...the account was not suspended, and the person had added books to his wishlist since the time the book was marked received.  A PM I sent to him was never responded to, and if it was even read, it was not read via the PBS website.  (I know some people just read the PM's through their email.)  My post office told me not to worry about paying the postage due when I offered to pay (and even they could not make heads or tails out of the multitude of stamps and labels on the book.)  I reposted it, resent it, and it has been marked received and I have gotten a thank you for the member.  Guess I will never know what adventures that book went on.

I suspect that the person who ordered the book had ordered it from someone else, and that person never marked it mailed and it rolled over to you. 

The person who ordered it got the book and marked it received from you, however it was the first book. 

They refused the second book (from you) and had it returned to sender.  The PO marked it Unclaimed rather than Refused.