How do phone alarms in shops work?

Okay first off let me say I’m NOT a thief, I’m just curious. Right so the other day I was in a shop and they had a fully operating iphone 3g, being held down by a piece of string. There were no alarms on the door and none on the iphone. I’m just saying there has to be a catch doesn’t there? I was wondering does anyone know what the catch is?

The string has a metal wire running through it. When cut, the alarm goes off.

MoMA Design Store Clocky Alarm Clock

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