One of those mysteries he'll never solve unless he asks Avery about it - that key. It's been missing for years. Even Albus couldn't remember the last time he saw it, and Scorpius didn't care then. Harry and Draco, softened by the Curse, the effects of which they had felt first hand, were downright respectful of their privacy. They never just barged into a room without knocking, so losing the key had no embarrassing consequences for them.