Surprise
Thanks to Ogawa destroying the wonderful mood that bubbled up between us, my time with Kazuha-nee ended there.
I had no idea if he was listening in on us but I’d give him the benefit of the doubt that it was merely a coincidence. Because if that's not the case, he would've acted more frantically after figuring out that I was inside his Aneki's room and we were alone together. He wouldn't knock gently and politely inform her of their parents’ message outside the door.
Besides, Kazuha-nee’s room has great sound-proofing. The only reason I heard her knocking on his door earlier was because she left the door ajar.
Maybe she did it intentionally to make me relax and not worry about Ogawa disturbing us.
When she returned, all sounds became isolated. Even the chirping of the birds or the whistling of the night breeze outside.