That’s not quite correct; there is no model with which the image is compared. The scanners work by bouncing either x-rays or millimeter waves off your body. The radiation easily penetrates your clothing and hair, but does not penetrate your skin, and the reflected radiation is analyzed to form an image of the outline of your body, against which items of a greater density than skin (be they metal or plastic) can easily be seen.
Here is an image from the Rapiscan (lol) backscatter x-ray systems that were deployed at airports (and were removed in 2013, for the vendor’s failure to implement a better privacy system).
The image cuts off at the edge of the skin. As you can see, the hair of the model is not shown. Any dense items that were located in clothing that extends beyond the edge of the skin, or in hair that extends beyond the edge of the skin, would not be imaged against the skin.
The millimeter wave systems produce a different sort of image - it kind of looks like an Oscar statue - but similarly does not show any image beyond the edge of the body or show the person’s hair.
In theory, these images may be simplified in the viewing booth, into drawings of a person’s outline. In practice, leaving bored people alone with computers leads to them figuring out how to turn off that feature.
Here is an explanation of how these systems can be beaten:
I’m sure you can see how the method this fellow describes - carrying an object of greater density along with his body, but not directly on his body - would apply to items carried away from the body in hair as well as items carried away from the body in clothing.
Not to say that any of this is good.
