Who all's familiar with the term "gray man"?
Short and sweet of it is that the term is synonymous with "anonymous", "incognito", "Jane/John Doe", that sort of thing. I've known the term effectively my whole life, and have used it and been understood when using it, so others know it as meaning the same as I described. Normally I'm good with tracing the etymology of phrases and words, but I'm lost on this one. I see a 1943 English drama called "The Man in Grey", apparently the "Gray Man" is a character in the Hardy Boys books, "Gray Man" is the title to a 2002 and a 2007 movie, and there's even an anime with the name or something near enough to it.