How does an anti-shoplifting scanner know if you’ve bought the item?

Question

I read that the scanners at the doors of a store detect items with magnets and such but what I don’t understand is how it differentiates between items that have been paid for

In: Technology

Answer 1

they can remove the actual magnet or demagnetize the magnet (ie in book bindings). typically, this can only be done by the cashier, so if you paid for it, you’ll walk out. if you didn’t or the cashier messed up, you’d sound the alarm.