Ron gave Hermione a rather dumbfounded look. "Yes you are," he insisted. "You're in all my classes." Because ever since their first year together, Hermione had been in every glass he was in with the exception of Divination. And he had dropped that class. So Ron just assumed that Hermione was always there. She did his work at the least. Or use to. Before this year when he started really doing his own work.
It wasn't that Ron didn't notice when Hermione wasn't around. He was just so accustomed to her always being around that maybe he took it for granted sometimes. Or all the time.