Shoplifting Question?

To be clear I don't steal or shoplift. When you go into a store. They have these things that make a loud noise. When you steal something, does noise thing scan the barcode on a item that hasn't been paid for? Like with those prepaid cards for itunes, playstation network, xbox live, and wii points. Does the noisy thing scan the barcode on the prepaid card if its not paid for?