Like the best martial artist, it is almost certain that the best chess player is undiscovered. They do not reduce themselves to flaunting their righteous skills for fame or profit. They live and breathe chess in isolation, playing simultaneous blindfold games against themselves in mountain caves.
best chess player undiscovered?

You mean like Sultan Khan who defeated Capablanca horribly, giving no counterplay at all? The game can be viewed in kingcrusher's channel.(if you want). Well, may be!

The odds that someone has had the time, materials (computer + software, books) and trainers to practice chess for multiple decades intensively every day, but has somehow evaded public attention during that entire time, are astronomically small.
Also, there's no sensible reason why someone would be really good at chess but not play the game in public; money and self-satisfaction are critical reasons why any professional chess player would not want to hide him or herself from the world.
Is it for sure or only likely that the best chess player is not discovered? such as someone from an eskimo tribe or other marginal society? I say it's obvious given the number of people in the world.