It's quite odd: in spite of some three years intervening since he actually had to interact with Granger, she still rubs him the wrong way. Perhaps it's that she sounds so damned swotty, or that she's off to asking questions already - or perhaps he simply remembers what a nuisance she and her comrades were - but whatever the case, the old feelings of sheer irritation and disgust are coming back to haunt him.

Like acid reflux.

Yes, that's precisely it. Hermione Granger is anthropomorphic heartburn.

He doesn't respond to her question; instead, he bites back his scowl and turns away, making for the street, away from the crowds. There isn't a glance back to see if she's following - falling back on old habits, really. He simply takes it for granted that she's coming because it's what she would have done as a student.