Hermione had gotten used to watching Ron over the years. In the early years it was to make certain he wasn't stepping out of line which turned into watching his goofy awkwardness. Now when she watched Hermione saw less of the boy she'd known and more of the man he was turning into.
When he returned with their drinks Hermione continued to watch him. He'd looked a bit surly as he'd walked back across and she could tell there was definitely something on his mind. "Like what?" she asked not quite sure what he was getting at.