Greta was taken aback by Victoria's instruction to knock on the door. Was she really being reprimanded like a student for trying to go where she shouldn't? Then she noticed Victoria drop out of a ready position and felt instantly ashamed at her own thoughtlessness.
Of course Victoria was right to be cautious. Greta knew that most of the Wizarding world had been fighting a war and she'd been told that the school was no longer as safe as it once was, but somehow the idea of that affecting her just hadn't registered.