The slight hesitation at the mention of her son drew his interest the most, of course, and he wasn't all that surprised to hear her comment regarding a man Severus knew for a fact to be anything but 'not so bad'. He wasn't about to point out such a fact at the moment, though. Instead he allowed her to grow more comfortable, taking careful note of both the obvious and subtle changes in her demeanor.
"You guess?" he echoed once she was seated upon the table, managing to just barely keep his displeasure at such a display in check. It wouldn't do to begin chiding her now. Even the smallest infraction and the girl was likely to flee. Instead he continued on the present course, something telling him that this impromptu, unexpected meeting might well prove useful at a later date.
"An odd choice of words," he continued smoothly. "Typically one tends to know whether they're residing at a place or not." A brief pause, one eyebrow arched, as he added in a tone that sounded like he was almost speaking to himself rather than to her, "Unless, of course, there isn't much choice given in the matter..."
He let the statement hang in the air between them a moment before moving on as though he hadn't spoken it at all. "So tell me, Miss Gaunt, how are you finding this fair, Muggle-filled city?" She had yet, of course, give even the slightest hint that she might be magical in origin, so in a way Severus was providing a rather simple test of her awareness skills.
Would she realize such a fact, and allow him time to explain how he'd come to a correct, yet what should have been impossible, conclusion? Or would she remain oblivious to such a simple, easily overlooked fact? It was more idle curiosity that caused him to want to know such an answer but he, of course, had far better reasons than that for having made the comment in the first place.