“How do you know if they haven’t ‘admitted’ it?”
Well, I could guess. But that’s not the point, my knowing. The point would be that it indubitably has happened in the past that atheists have both kept their beliefs secret and had certain beliefs without fully realizing it themselves.
Atheists don’t have beliefs, by definition. And they don’t feel the need to “admit it” because they don’t have guilt. I have never met an atheist who expressed guilt about it. Have you?