When do you fist suspect that something may be unusual or supernatural about mrs dodds?

In the novel "Percy Jackson and the Olympians: The Lightning Thief" by Rick Riordan, the reader first suspects that something may be unusual or supernatural about Mrs. Dodds when she turns into a Fury, a creature from Greek mythology.

This happens during Percy's field trip to the Metropolitan Museum of Art in New York City. Mrs. Dodds, Percy's math teacher, is leading the group when she suddenly transforms into a hideous, monstrous creature with bat-like wings and sharp claws. She attacks Percy, but he manages to escape with the help of his friend Grover.