12. My Very Own Hogwarts, School of Witchcraft and Wizardry.
Harry is the heir of Gryffindor (Well, Duh) so Hogwarts is his (not that he knows it). When he has had his magical maturity or he becomes 16, the wards go to him (thats how he discovers it, as he steps in the great hall a white beam of light strikes Dumbledore, a ball of multicoloured light comes out of him and goes to harry...) and he can sense everthing that happens in the castle. The power drain/transfer also unlocks centain things in himself/the castle.
Maybe because of that he realises several things. Like how all the things he can now sense about everyone in the castle Dumbledore just couldn't have not known about Quirrel being possessed by Voldemort, Ginny opening the Chamber of Secrets, the basilisk roaming around, Sirius in the castle (?), Crouch Jr. Posing as Professor Moody (?), Umbridge abusing students, etc.