In the The Horus Heresy - 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: did the Emperor know that Omegon was found? If yes, when? (as it seems the Alpha Legion kept it secret in Alpharius: Head of the Hydra)