That is odd.  I had one close to goign lost and when I looked it up it showed a scan of return to sender or something like that. I had mailed the book so I thought that was odd. I PM'd the person to warn the person. Well the book showed up like a day or two later where it was supposed to go. So I'm assuming someone hit a wrong button.  It's possible that an enroute scan was done by hand and misentered.  Or it's possible that the book was returned for inadequate postage or something and the sender just remailed it without bothering to tell you.    I would PM them ask if this is the case since it looks like the book is close to going lost. 

That is exactly what happened when I had a book returned to me. I blue boxed the book in the next town. It went by a moron of a worker and was sent back to me. My post office scanned at the unit in the am and delivered when they put it back in my box that afternoon.

I would say your sender received the book back on the 3rd.  I took it back into the PO the next day and it was sent back off  without needing to pay postage again so mine received additional scans.

With the time delay, I wonder if they got it sent back for trying to send with stale postage. If that was the case, they may have repackaged and the book is on the way without the DC getting scanned. I'd PM and ask if they've resent it.

It happened to me once as well. I was so excited to find a PBS book package in my mail -- and completely failed to notice the "to" and "from" addresses. Was I surprised (and disappointed) when I opened it to find a book that I had previously packaged and sent. It made its way from the east coast to the west coast and then back to the east coast because a postal worker inadvertently keyed in the sender's zipcode (i.e., mine) instead of the requestor's zipcode. I simply took it back to the Post Office and it traveled cross country once more -- and successfully arrived at its original destination. It was no big deal and still arrived at least 10 days before the "lost" date, racking up lots of frequent flyer miles! (But I did PM the requestor to let her know what happened.)

My books are almost always delivered the same day they are posted as "arrival at unit".