(𝙴𝙲𝙾𝙻) 𝙺𝚒𝚖𝙹𝚘𝚗𝚐𝙸𝚕 (kimjong_il)

Here's what I think happened: the old man heard the fight between the boy and his father a few hours earlier. Then, when he's lying in his bed, he heard a body hit the floor in the boy's apartment, heard the woman scream from across the street, got to his front door as fast as he could, heard somebody racing down the stairs and assumed it was the boy!