In WoL there is one named "THorner05S.SC2Map" which is the secret level for WoL.
If you look in the HotS map list there is no map apart from the normal ones.
So I found the 'secret documents' like there was in WOL however I didn't get anything after that mission happen about it.
So its the mission where you're going to rescue raynor from that space prison. There is part towards the end where you're entering a heavily fortified room and Vellerian and Horner chat to Kerrigan and she basically says about putting the vilephorm things to help bust through, in that conversation however Horner mentions getting the list of inmates in the prison, if you go back just before that room there is a passage to the north which has about 8 ghosts gaurding 'secret documents'
I assumed there would be something mentioned but there wasnt, I wonder if i had to restart and didnt collect them the second time or maybe I should check archives or something