Out and about today, and I found two of my releases... one that was released a block east of my house, and in 3 years traveled two blocks west. The other I released before Christmas, and it's been sitting on a ledge at the Safeway Starbucks ever since, so I picked it up, and will try and release it somewhere else. Obviously no one wanted them in those locations, right?
If it's a safe, sheltered location I might leave the book there, but otherwise I'd take it back and release it elsewhere. [Sometimes I spot books of mine in secondhand bookstores or thrift stores; in those cases, I usually leave them, but will make a note of the BCID so I can add the current location to the release notes. If it's a book that I know would work for an upcoming challenge or bookbox, I might even buy it back.]
It is possible that a book could have been taken, read, and returned to roughly the same spot; that happens at some of the local book-swap shelves. But if you're sure that the book's just been sitting there - or if you have a better release spot in mind - then go ahead and move it!