Auggie knows what he overheard. That Kate is trying to brush him off does nothing to assuage the concern that made him approach them in the first place.

"If you're talking about Joan, it's a conversation I want to be part of," he points out, clearly ignoring both of their assurances. Even if it isn't about Joan, any kind of disappearance can be critical to note when she still hasn't been found. Glancing toward Kate's direction, he asks, "Does he know something?"