Aidan looked thrown, because he really didn't expect a 'nice to meet you' from a guy he had tied to a chair. It made him instantly suspicious. Some sort of trick. The guy was trying to get him to let down his guard. Well, it wasn't going to happen!
He decided it wouldn't hurt to answer that question. "Because I've been watching you," he said casually. It didn't seem a strange thing to Aidan.