Earlier, Manju asked me what I thought of the look Naru gives Mai at the end of volume 5. I didn't have a firm opinion at the time. Now I do.
I guess that look over his shoulder could be an "I like to tease you" look. It could even be an "I like you" look. But neither feels right to me. I think the look is a "You just proved my point" look. The image of Naru feels too sad and disappointed to me for it to be either of the other looks.
Plus, Mai did just prove Naru's point. She automatically assumed the worst of him after his comment. What he said may not have been what people are socially conditioned to say at that moment. But that doesn't mean he met anything offensive about it either. If you think about Mai's personality, Naru is right. She's demonstrated his observation several times. And I think she did it again just then.