She also knew that it was likely still in the castle. Hermione had turned over the timeline of Tom Riddle in her head thousands of times since Harry had shared it with them, and she'd done some investigation on her own. Why else would Tom Riddle apply to be a Defense Against The Dark Arts professor not once, but twice? She highly doubted that he had any interest in shaping young minds, but she should see the appeal of it if you wanted to hide a horcrux.