She might not know Jamie overly well, but she certainly recognizes that sound. She's heard it often enough, though most of the times she's heard it back home had been linked to sorrowful occasions. As it is, the notes don't sound particularly cheery, so when she steps out onto the brisk battlements and finds the source of the music, she's immediately concerned.

Even so, she doesn't interrupt him, or even move directly into his line of sight. Let him play what he needs to play, and notice her in his own time. When he's ready to look up, he'll see Hermione standing there, looking at him with a concerned question in her eyes.