(Spoiler) how did marion know?

There were a few scenes where it became that clear that Marion's character figured it out. There was a scene when Louis and Catherine were discussing her children and the camera focused on Louis and it seemed that she picked up that he was contemplating something. Another give away was Louis talking about going to see the family's old house after they had lunch. He says he wants to see it out of nostalgia and it did seem random so maybe she picked up on that. Though to me the most obvious scene was when he was looking through his old stuff and reminiscing about his old boyfriend and Catherine interputs him. I think all of these scenes were hints that Catherine picked up on.