President Lee was still in his study when he heard someone mumbling through the crack he had left in the door. Earlier, he had gone into Mina's room to continue their conversation, but when he went into the girl's room she was gone.
Ian knew from the guards he posted at the doors that Mina had not left the house; which meant she was with his sister. He left his study door slightly ajar to catch Mina coming back from where she had hidden herself. All Ian knew was that he needed to talk to her.
From what Mina had said, Ian gathered that she did not believe he was also drugged that night in the hotel. He thought that she would at least trust him enough to believe that, but she did not.
That meant that Mina was under the impression that Ian had forced himself on her, and while he could admit that he was more willing than she was, they were both under the influence of an unknown drug, not just her.
