Shadows generally appear between the object which casts them and the light illuminating  that object.

Here, the shadow of the rectangle exists between the viewer and the hologram. Moving past the piece causes the shadow to shift and eclipse the object that caused it.

We have learnt to take the picture plane very much for granted. It is the surface on which images and marks are located using painted or graphic perspective, it becomes the ‘window’ through which we view the depicted scene.