Steph is trained to notice certain things, such as people watching her and combined with the sort of wariness that comes with being a female Gothamite, the random blond dude walking his dog draws her attention. He's the same distance away that she'd pick to covertly observe someone - close enough to actually see them, but not so close as to seem suspicious. She doesn't recognize him, but that could mean anything. He might be someone from the network, its not like she's been very careful.

She could just be being paranoid, though, so she doesn't actually do anything about him yet. Well, not much. She finishes the kata and goes into something more like tai chi; slow and controlled movements that make it easier for her to watch him watching her.