He must have known either after he was evicted that there was a secret room and "Legends" were coming in or heard Marcs voices in the diary room but didn't say anything.
Originally Posted by Borefest: “He knows Mark is coming back”
Yeah I thought that too. Last night when the four re-entered the house, the first words words out of Simon's mouth to Marc were "have you seen me all day saying nice things about you!"