Forgotten Secrets are memories so dangerous that Councillors have to purge them from their minds and store them in caches so their guilt doesn't shatter there sanity. Until a secret document was given to Sophie by Lady Gisela, Vespera was part of a Forgotten Secret. Consequently, only the guards that tended to her in prison in Lumenaria knew of her existence and who she was.
There are about 50 Forgotten Secrets currently known.
Additionally, 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 (Neverseen).