"What are they doing?"
Unbeknownst to the two, Natsumi actually found out that Hajime and Megumi went out together for the last day of the school trip, but she didn't think much of it since she knew they were friends, to begin with. While most of the students in Dai Yama High School had no idea how close Hajime and Megumi actually are, Natsumi could more or less already sense the chemistry between them. Even before they 'accidentally' visited Hajime at home, she had already known that they were close.
After briefly letting everyone in their group know that she'd be separating from the group, she immediately started tailing the group, assuming Megumi's usual job of stalking.
