A visual image for the 'unknown unknowns'. Imagine the mind as a sphere. Points of understanding emanate out from that sphere in all directions as beams of light with varying diameters. Each point of understanding is potentially infinite, just as the sphere is potentially infinite. These radiant points of understanding increase in area relative to the plane of the sphere as they go out from the sphere, like expanding cones, but the angle of increase is less than the increase of a cone the base of which would be at the center of the sphere, and the arc of which would be defined by its points of intersection with the sphere.
In other words, if that part of the sphere that intersected with these beams of light had expanded according to the natural arc of the sphere, the cone would be larger. Thoughts, while infinite, are directed outward from the subject, and don't have the native fullness of the mind.
So we have these thoughts in all directions, but as they travel outward and grow large themselves, making conceptual syntheses possible with other beams of light coming from the sphere -- not crossing, but in relation to each other, an even greater area of darkness is created between them. Perhaps this is the area of the 'known unknowns.'
The 'unknown unknowns' perhaps come from the fact that our thoughts and representations don't retain the native arc, the original angle of the expanding sphere, because they are directed towards the object. This would be the difference between the area that would be covered by a cone that retained the native fullness of the arc of the original sphere, and the focused beam, which although expanding and infinite, expands to a lesser degree. So the known unknowns are those areas outside of the boundaries of our thoughts, and the unknown unknowns are concealed between these boundaries in a sort of a penumbra around each notion due to the representational nature of thought itself.
I know what I know, and know what I don't know. But what I don't know due to the act of knowing is hidden.
Perhaps. Just a rough notion. Not particularly promising, quite yet.