i know it,first of all,when you begin to the first or second stage,try to search the hidden place of the chaos emerald,i can't rembember the bridge part but the cemetery part there is a moment where you pass a little and come back to see a crypt opened,there is a hidden place with a bed heart-shapped and the key pieces guarded by a succubus and a incubus,when you kill them,you get all the key pieces,continue to get inside the mansion,there is a door with a keyhole,if you get the two key pieces from that crypt you can opened it,inside there is these two ghost who working with the ghost lady cosplaying something and there is that ghost lady's diary and the chaos emerald of the stage four.
is probably they change that location of it in the new version so get it in this version so you can see the diary of this version before it get changed.
i think i hear or see they going to remaster this game in a 3d beat em up version like in final fight streetwise from ps2 (saddly... i just played the ps2 version only) so is probably everything we know so far of the secrets places,everything is going to change in the new version like locations and etc...
i imagined they doing the 3d remake of this game because some peoples enjoy stealing sprites of the game to do others hacks games or just using the sprites without permission but we don't know about if that a rumor or is real so yeah,let's wait a little more.
