Okay, this one is really cool.
One of the “Hidden Christian” identification techniques in Japan was to use a “magic mirror” (makyoh). These were metal mirrors designed to show you looking at yourself with great fidelity; but if light bounced off the mirror onto a surface, tiny irregularities in the smoothness of the front (created by its hidden molded metal backside, which was further hidden by a case on the back) will make the hidden image shine out onto that surface. The usual Hidden Christian design was a crucifix.
Site selling a reproduction Catholic magic mirror. (Mouse over the picture.)
UPDATE: Welcome, Mark Shea readers!
Oh, and I forgot to link the makyoh videos. Here’s a video from the “What the Ancients Knew” show that I saw. Here’s one without explanations that’s very short.