Severus doesn't startle, but the way he turns his head just so to react to Njoki addressing him says he didn't expect her to add anything to him. (Or maybe he just didn't expect something like a thank-you?) He doesn't scorn it, though he seems a bit wary. It's probably not personal, this guy isn't screaming 'conversationalist of the year'.

"Neither have I," he admits. "But that kind of enchantment, on anything, has a particular resonance."