is it correct to use "behind" and "back" at the same time?

I understand if something is done behind you, it is already unknown to you. Your back has no eyes and therefore one cannot see what’s done at their back. I am driving towards the use of "behind my back". it sounds like the opposite. for instance, if someone says the pen is "behind the back" of the book, the person may literally mean the pen in front of the book.