The bar code wold contain information like the description, price & weight.<quoted text> Ever hear of a bar code?
When you scan an item, & you put in in the bag, the weight is checked to see if it matches what the bar code says.
I think you are referring to an EM strip that is placed on certain products that must be demagnetized at checkout or a scanner will pick it up when you leave the store.
Only certain products are tagged in this manner. If you watch the check out clerk, you will see them run certain items over the demagnetizer.
I do not think this can be done or is being done thru a bar code scanner.
Self check out also makes using stolen credit cards much easier. There would be a risk if I gave a clerk a credit card with the name Josephine Flack but the self check out wouldn't care.
As self checkout becomes more important, I'm thinking an cell phone AP that provides the weights of products would become very popular among thieves. You scan in what you want to steal he AP returns what other products have the same weiht but cheaper in price.
We can stop by the self checkout beer distributor & scan a case of Old Milwaukee but take a case of the good stuff & pay with Carol's credit card. Yee Haaaaaa