The wand listens for a metal object’s reply
Inside a handheld detector, a coil carries changing electric current and produces a changing magnetic field. When the wand passes near a conductive object, that field can drive tiny circulating currents in the object. Those currents create a weaker magnetic field of their own, which the detector’s coil can sense.
The wand’s electronics compare the returning signal with the field they sent out. A difference suggests nearby metal, and the wand turns that change into a beep, vibration, or light. The object does not need to be large or magnetic: conductive metals such as aluminum can respond too.
Why small things can set it off
A thin foil wrapper may seem insignificant, but it still conducts electricity. If it is close enough to the wand, its induced currents can produce a detectable response. How noticeable that response is depends on details such as the object’s size, shape, orientation, distance, and the detector’s sensitivity.
That is why a security wand can reveal an overlooked scrap of foil as well as a larger metal item. It is not recognizing what an object is; it is detecting how the object changes the electromagnetic field nearby.
