Lucius is not looking at the door, having been momentarily distracted by a nearby noise. As he hears the door open, he starts to speak as he turns.
"Ah, good, Professor. I was wondering..." his voice dies off as he stares at the figure in the doorway, his eyes widening with shock. Before anything more sensible can come out, he mutters "Merlin's beard! Severus??"
Oh dear, Lucius just might pass out. I think he thinks he's hallucinating, as in his world Sev has been dead for several years.