I think. It's that part when Tes told Ailette to explain about the sea dungeon. He heard Agnes saying all hobby is fine but maybe talked about the dungeon a bit? He used Ash there an excuse.
Ohhh!! That actually makes sense ;0
Our Ailette so smart. Clocked it so quick
There are a few more SUPER subtle clues quite infrequently alluded to in the novel after the tutorial ended, but this is/has been the biggest tell of them all. It was no wonder she eventually asked about it. Its kinda implied that Tez tried to keep that a secret so he could basically eavesdrop on their conversations since Agnes clearly knew MUCH more highly important info that Ally was purposely keeping from him.
Genuinely confused on how Ailette came to terms that Terry could hear Agnes. I never noticed