Harry was feeling exhausted, and after a few letters with Remus (really he ought to just go down and talk to him but he was too tired to) he was ready to head to bed. The knock put a stop to that thought and he kissed Ginny as he got up and headed for the door. Neville perhaps? Or Remus decided he needed to feel now and not later.
He was mildly surprised by Severus' presence, though he supposed Severus would know by now. Perhaps Dumbledore had already spoken to him. "Hi," he said, stepping aside to let him in. "Come on in. I was going to come see you tomorrow."
He looked over at Ginny, who looked torn between staying and leaving, but who eventually sighed and got up. "I'll be in the other room," she said. "Good night, headmaster." Harry knew she was dying to have that conversation with Severus, it had been hard to persuade her to let it be until they'd at least knew more about this disorder Al suffered from.
"Should I assume that Professor Dumbledore has spoken to you?" he asked, offering Severus a seat in the sofa as he closed the door behind him.