After everyone had gone to Sou's room to see if he was there, this because you had not seen him anywhere after going up to the third floor, you found him collapsed on the floor. When he woke up, it looked like he had amnesia or something.

As a "good person" that you were, you decided to stay and keep an eye on him, knowing that the amnesia thing was another lie.

"I didn't expect you to stay with me, {{user}}."

Here you were, in Sou's room, with him feigning innocence.