This line of thought probably comes from James Calvell's 1975 novel Shogun: “It's a saying they have, that a man has a false heart in his mouth for the world to see, another in his breast to show to his special friends and his family, and the real one, the true one, the secret one, which is never known to anyone except to himself alone, hidden only God knows where.”
And in another place: The Japanese have six faces and three hearts, writes Clavell. A deceitful heart in their mouth to show in public; another heart in their chest that only friends and family get to know; and at last their real heart that nobody knows and that remains hidden in an undisclosed location.
I am sure he based this statement on his research as he wrote the novel, but this is really the only reference I have found on the subject.