Settled in the story
Where was Ravenclaw's diadem before anyone went looking for it?
Harry walked straight past the lost diadem while hiding a textbook, close enough to use it as a landmark. The trail covers the tiara on the bust, the Grey Lady's confession, and the fire that ended a hiding place a thousand years deep.
This thread gives away a resolved story point
The clue
A tarnished tiara on a statue's head in the Room of Requirement, passed at arm's length in a hurry — one book before the diadem became the last secret of Ravenclaw Tower.
The trail
- Book 6 · Sectumsempra
Needing to hide the Half-Blood Prince's book where nobody will ever find it, Harry uses the Room of Hidden Things and marks the spot so he can find it again: a great cupboard, and beside it the bust of an ugly old warlock wearing a tarnished, tiara-like crown. He even settles it more firmly on the statue's head. It is set dressing. It is also the diadem.
- Book 7 · The Lost Diadem
Back at a wartime Hogwarts, the search for something of Ravenclaw's narrows to a lost diadem that nobody living has seen. The Grey Lady's story supplies the shape of the thing — and Harry's memory supplies the address. He has been carrying the hiding place in his head for a year.
- Book 7 · The Battle of Hogwarts
The Room of Hidden Things gives the diadem up in the middle of the battle, and Crabbe's Fiendfyre does what centuries could not: it burns with the rest of the lost things, one Horcrux destroyed inside the very room that hid it.
The payoff
The diadem of Rowena Ravenclaw — stolen by her daughter Helena, hidden by Tom Riddle in the one room he believed only he had mastered — spent a school year on a bust's head in plain sight, marked out by the one boy who would come back for it.