This article contains plot details from the latest book.
The caches are stored in the void. Until a secret document was given to Sophie by Lady Gisela, Vespera was part of a Forgotten Secret. As a result, only the guards that tended to her in prison in Lumenaria knew of her existence and who she was.
Councillor Oralie gave Sophie the late Councillor Kenric's cache just before Sophie and her friends left the Lost Cities to join the Black Swan in Alluveterre. Kenric's cache contains seven Forgotten Secrets. Fintan's cache contains nine Forgotten Secrets. In Neverseen, Keefe mimics Sophie's voice to recite the password needed for him to get Kenric's cache, then he gave it to Fintan as an attempt to gain his trust. Then, in Lodestar, when Keefe returns from his time with the Neverseen, he endeavored to steal back Kenric's cache, along with Fintan's cache (that he had from his time of being a councillor) as well. It is later discovered that both of the caches that were recovered are fake and don’t actually contain anything.
At the end of Legacy, Oralie tells Sophie her theory that her cache's sole Forgotten Secret has to do with stellarlune. The two of them agree to work together on opening the cache.
The cache was opened in Unlocked, the password being "Fathdon". The memory was of Oralie encountering Kenric, and him saying something about Elysian and then pressuring her to tell him more things.