Librarians do hide books. But not as a way of hiding from public but as a way of not throwing them away. Let me explain. Even though libraries are very big still they run out of space and regularly throw out (in India because most don't care) / ( or in Texas sell out books cheaply) and it pains the librarians deeply so they kind of stash books secretly from being thrown away. And if you really show interest in a particular book and request it nicely then the librarian will give it to you and tell you not to tell anyone. Why I know this because it happened to me in my college library in India. I can still after decades remember the love of books on that librarian's face
Libraries remove unborrowed books and sell them. Borrowed books stay on the shelf. That's what they do with books.
Judging books by their unborrowedness is like judging a youtube video's educational content by its view count.
It's a bad reality created by the powers that be