Hermione had narrowed her eyes because she was not naive enough to believe anything Zabini had just offered, at least about Charlotte Dullys, who happened to be the Ravenclaw in question. But Hermione had considered the fact that Charlotte had successfully confiscated something from Zabini and gotten away with it hex-free to be a statement to her character. She briefly considered that if she had been able to manage that, she would surely be able to hold her own against a flirting Zabini.