Tom had a habit of going down to the chamber fridays after Slug club meetings. Since Hermione had been quickly added to Slughorn’s collection of star students she easily followed Tom one night under Harry’s invisibility cloak. When he jumped down the shoot behind the sink she was quick to follow before it closed behind him. She was immensely pleased at the sight she found at the bottom of the shoot. Harry stood, wand trained on the neck of a petrified Tom Riddle. They knew that if they were to follow Tom into the actual chamber they would have to face his Basilisk, so they captured him in the corridor before the main chamber to keep them safe.