So the person who rings the doorbell was a deliveryman. To what I remembered, inside the envoloped was Inseop's passport. When the ML got back to their room, he found the envelop (?) so he thought the MC run away from him. He went crazy and tried to look for the MC all over the place/city. But just went out because he bought a cake for the ML. I think it was his birthday that's why. 

But when the ML found Inseop he didn't even give him a chance to explain himself and grape him to the point his an*s bleed.... :')