How does the theft detectors work at barns and nobles?

I was curius to how the metal dectectors at Barns and Nobles and other stores like that work. I have look through books magazines ect and I can see no visiable strip b ar that would set one off. Its a learning experience to me. Is it something in the mecanics of the dectector or computer related?How does the theft detectors work at barns and nobles?
They put the rfid tags in the spines and covers of the books at the printing press now. But even if you pull it out you will never make it out of that store with those five finger discount erotic photography books -- the rfid tag in YOUR spine will detect your theft thinking patterns and alert the authorities. How does the theft detectors work at barns and nobles?
RFID tags and chips

the bees