Hum. Makes me think. The Silent King was in selfimposed exile in the void, until he discovered the Great Devourer and came back to the galaxy to set up a defense against it. He claims he met Sanguinius and tried to warn him about the threat, and had a death mask forged in his likeness. Taken from the short story The Last Word of the Silent King.
I daresay that the Pharos event might have woken up Szarekh as well and having an encounter with the devourer, before fleeing home. And comming to where he sensed the Pharos event.