Hirata Estate appears twice in Sekiro — once as a memory accessed through the incense box early in the game, and once as a different version of the same memory accessed through a different incense item found only after specific late-game quest conditions are met. The second Hirata memory is not signposted. The player must know to look for the alternate item.
The second Hirata memory shows the same location in an earlier time period — before the fire that destroyed the estate in the first memory — and features Owl, Wolf’s father, as the primary antagonist. The memory reveals that Owl planned to betray Kuro long before the events of the main game, that the fire at Hirata was partly his doing, and that a figure connected to the Shura corruption was present at Hirata’s founding.
The Owl Father fight is widely considered Sekiro’s hardest optional boss — harder than the Guardian Ape, harder than Genichiro, using a moveset that requires unlearning the defensive habits the main game builds. Owl fights like Wolf fights, because Wolf learned from Owl. The tells are the same; the speed is higher; the punishment for reading the fight correctly through the main game’s lens is death.
FromSoftware designed a hidden memory that requires knowing what the first memory was missing, finding an item that presupposes awareness of a questline, and fighting a boss that requires dismantling the game’s taught patterns. It is the game’s thesis on mastery: understanding something is not the same as being able to apply that understanding under pressure.

Found this by accident on my third run. Came here to understand what I was actually looking at. Great write-up.
Saved this article for my gaming reference folder. Essential reading for anyone serious about this game.