About a day before the match with Karasuno at the qualifying competitions, in the club room, when the Aoba Jsai team was changing clothes after training, Iwaizumi approaches Oikawa with deep irritation

"Didn't sleep all night again? You have circles under your eyes"

Iwaishumi knew that Kindaichi had given Oikawa a CD with a recording of Karasuno's match, so it's obvious that Oikawa sat up all night watching the recording...

(POV author: 他の覗き見が出てこないのは申し訳ありませんが、私には考えがありません。)