In the same way dryers seem to eat single socks, I suspect airplane seat-back pockets of an insatiable appetite for the small items we stash as we're settling in onboard. I've left books, magazines, and toiletries behind after forgetting to check the seat-back pockets before exiting.
But from the sound of USA Today's Gene Sloan article on reclaiming items left behind on flights, I'm a total lightweight with my lost paperbacks and moisturizers. People regularly leave behind big-ticket items such as cell phones, medications, and jewelry. Yikes.
This short and very useful article details the policies of reclaiming these items if you lose them. It also reveals the eventual fate of unclaimed items: the salvage company. The lesson I learned from this story is that if you do leave something behind, call the airline as soon as possible, and be sure to follow up if they suggest it. There are systems in place to route lost items back to passengers, but they only work when we, the seat-back-pocket absent-minded, come looking for our goods.