As far as i'm aware, they just have a thing in them that you can't not scan something and add it to the bag, however, they can't tell what it was or was not that you scaned. A mate of mine, used to do his shopping by going to the reduced section, removing all of the car code stickers and then go around and stick them on steaks, etc etc etc. Then just go and scan it all through the self scan, and get it all or pennies. If caught, he would just claim that it must have been some dumb employee that reduced the wrong stuff.