Question about hidden security device in retail packaging

[jimdagys]

I bought a box of 12 Dove chocolate bars from Walmart ( cashier took out the 12 chocolate bars (from the retail display box) and scanned each one, did not scan the upc on the box) and when I went through the security gate, the alarm went off. The employee at the gate took the empty box ( without the chocolate bars) and quickly ran it over a device, then I was able to leave the store without the alarm going off. When I got home I spent several hours very carefully inspecting the empty box (thin cardboard) , then finally ripping the box to pieces and separating the layers. Bottom line, I could not find any evidence of security device. My question is: How could such a security device be completely invisible? Is the security device printed on the box with invisible ink?

[transporterfan]

Magnetic ink? Like on cheque books maybe.

Edit: Nope. It's RF labels, probably a fake barcode sticker:

http://www.theftcontrol.com/Products-Security_Labels.html

[jimdagys]

Absolutely no sticker of any kind on the box.

[DB83]

It's fairly obvious that if the cashier could have scanned the box she/he would have. But I guess the box originally held more than 12 pieces. And why would you need the box anyway ?

It's probably not a sticker but more likely to be part of the printed packaging itself.

But who cares ?

[TreeTops]

Well I always thought it had to be a chip of some kind. Very interesting that they can hide RF data in a the printing.

[jimdagys]

And why would you need the box anyway ?

I wanted the display box to keep the candy bars stored in a convenient way. The cashier didn't scan the box because it was an open retail display box.

EDIT: On a side note, it is fairly obvious that when an item is scanned (the upc) by the cashier, this will deactivate the item and keep the security alarm from going off at the checkpoint. (no need for the cashier to run the item over a demagnetizing device. ) The only reason the cashier would need to run an item over a demagnetizing device is when the upc on the item is not scanned, as in the above example).