I searched all of the source for "hidden" to be absolutely sure and there are no hidden hiddens hiding hid.
@firstname I'm afraid I don't know what you could be referring to. The reason you might want to use a hidden folder to store your documents is so that another program won't index them. On Kobo if the default software (NIckel) sees new files it starts an indexing process while locking you out. Whether there's any point to using hidden folders depends on whether Kindle does something similar.
KOReader doesn't care either way.