Anyone know how library alarm systems work?

Okay guys, I'm not going to steal anything, I just want to know, how do they work? We have a system at my school where if you walk out the door with a library book the sensors go off, but how are they connected to the book itself? Is it something to do with the barcode, or are theere built in microchips( I highly dout that, I go to a public school) anyone know how they work? Thanks!