The only reason Avery might have spoken up would have been concern, and most of the time he wasn't able to override his own insecurities sufficiently to express it. In this case, with someone he barely knew, he didn't feel too bad about it. He felt like those in charge of this experiment might watch someone like Hannah a little more closely, anyway.
He was relieved that she was leaving, because he'd been dreading getting up while she was still sitting there, explaining what he was doing, maybe dropping things and being a klutz. Even though she couldn't see him, she would have heard it. "G'night," he said softly, deciding he was going to stay put until she was gone.