She did look a little bit familiar. Maybe he'd met her during one of his many, many, many evenings at The Leaky Cauldron with Thomas? His confused look melted just a bit as he said, "Do you spend much time at The Leaky?" It felt a silly question - a lot of people spent a lot of time at that particular pub.
He looked behind her before back at her face, taking a closer look. She really did look familiar. "Wait a second, you're on WeeVee," he added, suddenly recognizing her from Grace of St. Mungo. But that wouldn't explain how she recognized him...