It’s certainly enigmatic, but after reading the Byzantine history of the Mandylion, which many believe was the shroud, I believe the prototype was a piece of statuary that lay concealed for five centuries, covered with the cloth. Ionizing radiation from radioactive elements within the statuary, over time, caused the changes in the cloth that we see as the image.
Wouldn't this theory require that somebody, centuries ago, understood that ionizing radiation could potentially cause an image to form on the cloth over a long period of time? And what was their purpose for doing this? Or is it your view that the image was formed accidentally, on a cloth that was draped, coincidentally enough, over an extremely realistic sculpture of an individual who appeared to have been crucified?