An open secret is something which is widely known to be true amongst people, but which is denied officially, for that very reason, providing sources is more usually than not possible for an open secret.
I have no idea if an open secret is like the open mystery of Ephesians 3:9,10.
Indeed. That seems like a loaded cultural reference point considering the majority of the world (including Christendom) do fine not keeping up with bronze age letters as jargon. Maybe they mean an open secret like the Panama papers?
I have no idea if an open secret is like the open mystery of Ephesians 3:9,10.