The Headmaster was standing in the middle of the empty Entrance Hall in almost the same place he was the night she and her husband rushed to the castle. Evidently, he was waiting for her, prepared to face down a potentially irate parent. Wards surrounding the grounds must've alerted him to an additional presence that didn't belong there and it was a long walk from the gates. He would have plenty of time to reach the Entrance Hall before his visitor did.