Not all items will do that but higher value items that are more likely to be stolen have barcodes with electronics on the back of them. When it's active, the detectors will beep when you lass through them. When you scan the item, it also deactivates the electronics and letd you walk through without sounding the alarm.

When I was younger I used to take the electronic barcodes off of items and try to land them in people's bags or their jacket hood so the alarm would go off when they leave.