“. . .when you're on the train home tomorrow, you're so -"
But what they were, they didn't find out. Hermione had turned to the portrait of the Fat Lady to get back inside and found herself facing an empty painting. The Fat Lady had gone on a nighttime visit and Hermione was locked out of Gryffindor tower
(Harry Potter and the Sorcerer's Stone)
Is the prepositional phrase a predicative expression over the Fat Lady, or an adverbial modifying the verb?