Right right right. He's the one with the perfect memory retention. I'm suspecting that one of two things happened during Dan-I's disappearance the previous year. 1) He didn't forget her like the others because of this perfect memory(like some other readers are suggesting) or 2) because he knows he has perfect memory and thus would never forget someone who's contact information he has, he realized something was wrong and tried to contact her. My bet is on the second option. As for why he kept calling her for 6 hours straight, it's likely that he realized how important it was for him to get in contact with this "unknown person" he found in his contact list, and the fact that they didn't pick up no matter how many times he called probably made him feel anxious(I'm mostly just guessing here, though). He's already made it clear that he knew there was more to Dan-I than she let on and that her situation likely isn't exactly normal, but he probably didn't see just how serious her situation was till now. Or, if he had realized how serious it was, he kept quiet for her sake since she never went out of her way to share with them whatever problem she was going through and thus didn't want to scare her away or make her feel uncomfortable by pressuring her for the information. Hard to say for certain what is going on in his head, but I guess we'll figure that out come next chapter.