The Emergency Access Door is the Sazanami fail-safe: if the Storehouse owner dies before ownership can pass at Naginojoen, the subspace can still be entered. Inside the Kura it is a floating door. In the world it sits in a sealed cemetery room. Chihiro scouts the Storehouse through Enten’s residual charge and notices the door that is never opened. When he, Shiba, and Hakuri reach Naginojoen, the twin is already destroyed.
A door that is a plot
The Storehouse page is the island. This page is the lock the infiltration was aimed at. Kyora does not leave a spare key for a discarded son. Hakuri becomes a second Storehouse instead. See Storehouse, Naginojoen, Rakuzaichi.