Bring the plastic outside on a sunny day, hold it horizontal, and see how the sun reflects off of your scratches. Experiment with turning the plastic. Can you make the image appear and disappear? If you used the same center point, you will see a straight 3D line composed of one small dot for every scratch. Do the larger radius scratches pop higher above the plastic?
Try closing one eye and holding a finger next to one spot of light. Now close the other eye and notice the distance between the spot and your finger. Did you notice that the spots on the larger scratches are further apart? These two spots are the "Stereo Pairs" that are creating the 3D image.
If you saw the spot in the same location with both eyes, it would appear to be on the surface of the plastic. Your brain uses the differences between the two images from each eye to judge depth. Have you ever held your finger in front of you and noticed how its position seems to change as you blink your eyes?