Well, the answer of random stranger (and by that she meant muggle) was out because the bloke knew her name which meant that he was either a fan, a reporter, or a photographer that was looking for something. She was willing to stop for a moment to give an autograph, because generally fans meant well - especially if they had stuck with the Harpies for a year where they were unable to play. But she wasn't going to forgo her plans for buying Christmas presents for a reporter that wanted an interview or a photographer that was going to hound her for photos of James.
And then she processed what he had said, stopping abruptly in the middle of the sidewalk and causing two men to bump into her and glare as they moved past. Once they were gone, she whirled around and narrowed her eyes on the man, "Am I supposed to know who you are?" she asked, keeping her tone neutral for the moment. She didn't she a hint of a wand in a pocket, but they could be hidden easily enough. Ginny wasn't going to offer an autograph or bring up quidditch until she was completely certain the man was a wizard.