She had been ensconced in her book, and snaps from it with what's not quite a jump, instantly expecting some kind of punishment. It really does border on ridiculous, that particular Pavlovian reaction. It certainly makes her feel like an idiot.

What on Earth is he doing here? 'Snape' and 'festivity' are two concepts which don't seem to mesh in her mind, no matter how she tries to wrangle them into a logical connection. "Sir," she says, a little tentatively, having to force herself away from 'Professor'. Reminding herself that she is not his student, she adds; "I didn't expect to see you here."

She sounds rather wary; the question of why he's here talking to her is clearly implied in her tone. After all, she highly doubts he just wants to wish her a merry Catenrat.