When the lady smiled, Severus found himself becoming somewhat more successful in returning one of his own. Especially when he realized that her aborted action -- before holding up her hands in a non-threatening pose -- seemed to indicate she was in the habit of reaching for a weapon herself. Interesting.
"I'm fine," he answered with a nod. "I am, ah, simply not accustomed to finding myself with company in a library." Because he'd normally used the Hogwarts library after hours, if he possibly could. Best to avoid unpleasant encounters with students. "But I do thank you for your concern."