There's something very familiar about that peremptory, object-more-solid-than-knuckles-on-wood knock, but the Professor is out with a pair of exceedingly strange children and had only ordinarily locked the door on her way out, so Severus answers it anyway. And his eyes blink wide. "...Lucius?" His voice is audible, but only because he's still got a sonorus on it. With the state of his throat, he can currently barely whisper.
At this point, Sev is still a complete young scarecrow, but he's acquired the sort of stone-and-green soft shirt and sweatervest ensemble he used to wear around the dungeons.