What makes store theft alarms go off?

I'm just curious. I was shopping today, and I was looking through a rack of clothing that was near those detector things at the front of the store, and the alarm went off, but the clothes didn't even have those plastic things on them that need to be removed by the cashier (we call them no-gos at the Jacob... show more I'm just curious. I was shopping today, and I was looking through a rack of clothing that was near those detector things at the front of the store, and the alarm went off, but the clothes didn't even have those plastic things on them that need to be removed by the cashier (we call them no-gos at the Jacob Outlet I work at).

Is it the price tag with the barcode on them that sets the alarms off? Could it be just the simple barcodes on the price tags that set off the alarms, or some other anti-theft method?
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