Hermione's eyes flick to the wand, noting it but not commenting. She assumes he's casting the Muffliato charm; it is his, after all, and it's what she would have done.

The accusation-slash-statement surprises and confuses her, and she frowns, wrapping her arms tightly and defensively around herself. Something in his tone- or perhaps just an automatic reaction to him- makes her feel like he's trying to find something to charge her with.

But that's absurd. She isn't a student anymore. So: the Protean Charm. What use could that be to him? Her mind flicks through possibilities as she nods slowly, and says warily; "I did, yes." She doesn't have to say 'why?'. It's already in her tone.