See, sometimes our books go missing. And while we'd much rather get the originals back, sometimes books have been missing for so long that we eventually just need to replace them. (Or, you know, sometimes accidents happen and they need to be replaced due to damages.)
So if you've been asking us about a book for months and hearing, "Hmm...it should be on shelf..." or "Sorry! That one's been out for...gosh, almost a year now!", take a minute to check back in with us. It's possible we finally have it back on our shelves (unless someone's already beaten you to it!)