In the story it seems like Ennis might realize Alma saw them--or at least that possibility is open. Proulx says something like, "Alma looked out at Ennis's straining shoulders and shut the door" and later, "He was glad the light was dim on the landing but didn't look away from her." Hmm, now that I say this, I guess it's not really clear. I always assumed that he thought maybe Alma saw them--after all, she acts pretty strained toward Jack the whole time. Interesting speculation, though--and a sad commentary on their relationship that she never says anything to him until they're divorced.