Formal Investigation

"Then I don't know." Ron shrugged.

On the first weekend after the holiday ended, Harry and Hermione spent an afternoon sitting in the library.

Harry wrote down the known information about Paloma Observatory on a sheet of paper and listed Delaney's main movements at Hogwarts beside it,

including the date he first appeared in the fourth-floor corridor, the number of times he had been recorded by the Marauder's Map, the dates and durations of his visits to the underground area, and the date the Stone Platform room had been discovered.

Harry placed his phone on the table and checked the tracker mark's real-time location. "He's still in the castle, near the fourth-floor corridor, but not outside that abandoned bathroom—he's at the other end of the corridor."

"Maybe he's checking certain details along that corridor," Hermione said. "If he really is tracking the path of that erased line, he may be testing the signal strength at different points in the corridor to determine where it starts weakening or strengthening."

Ron leaned over, holding a thick book with a dark green cover and a worn spine.

"Found it—the supplementary record on the castle foundations from the school history room. Someone reorganized the foundation blueprints in the nineteenth century, and one page mentions the corridor beyond the iron gate in the southwest corner."

He opened the book to the bookmarked page and set it on the table.

It was a page of handwritten notes, the neat handwriting faded to light brown ink.

The note read: "There is an early passage beneath the foundations in the southwest corner of the castle. Its date of construction is unknown, and its purpose is unknown.

The passage was sealed during foundation reinforcement work in the eighteenth century, and an iron-barred gate was installed at the entrance. The