I wonder when we’ll see the first ‘spiritual light’ scam? Implant a small LED under the skin, develop a patter for the marks, and a new scam is born. There are many options for powering and switching the light.
And one of these folks will come across fluorescent dyes. Apply to skin, add in a black light (in normal daylight it wouldn’t be noticeable) and you get a hint of a glow.