The fallacy in the woman’s surprise is due to the fact she can’t see herself.
She concludes since she can’t see through the towel to the mirror’s surface, then no light is getting to the mirror’s surface to transmit her mirrored image. She is ignoring the fact that the towel is not blocking the entire surface of the mirror. She doesn’t understand the fact that light is still reflecting off her face and body, radiating toward the surface of the edge of the mirror toward the observer and his camera off to the side.
Her assertion that the mirror doesn’t know she is there on the other side of the towel, is 100% true for her own view with her own eyes. The observer filming this is not obstructed by the towel. He can see the mirror and her reflection caused by light coming off her and hitting the edge of the mirror.
This is a trick. It is just science.
The towel is not obstructing the view of the observer, only the view of the woman. His perspective is not her perspective.
As regards her “how does the mirror know I am there?” is true from the viewpoint of her own eyes. It is false from the perspective of the observer filming from off to the side where the towel is clealy not obstructing HIS VIEW.
No trick needed. Just physics.
Those people who are mystified by this probably can’t figure out why the dog can snatch up a rabbit that turned sideways to the dog.