there's missing context in the manga version.
in the novel, hikari actually moved rooms to her sister's a few years ago because her sister's room was bigger. thus, she no longer lived in the room across from her childhood friend.
thus, her childhood friend is used to not having her on the other side of the window anymore (explains why he didnt check 💀💀💀)
however, she just moved back into her old room (so that she could look at him lmao) and was quietly reminiscing their past and thinking about how to casually tell him that she moved back into her old room. unfortunately, while looking at his figure through the curtains...