In the primarchs series, in the book about Alpharius, we learn that Alpharius is the first primarch found (but kept secret).

Later in the book, Alpharius finds Omegon. His legion is now complete. However, it is not said if the Emperor was told about this. Moreover, considering the nature of the Alpha Legion, I would think it is kept secret even from the Emperor himself.

So my question is: has the Emperor ever known of Omegon being found ? If yes, when ? (as it seems the Alpha Legion kept it secret in Alpharius: Head of the Hydra)