They are in every dungeon, but there are low chances of finding one. you see, on every floor of dungeons, there is a secret stairway, but it's invisible until you step on it. This is the stairway to the secret Bazaar.
you must be getting it messed up with Pokemon ruby version. there is no secret slab in Pokemon mystery dungeon. at least in red or blue.
it lets you find extra legendary Pokemon in dungons
in mystery jungle after you get secret rank
he is in amp pains if you have a secret slab and a mystery part
It helps you get all of the legendary pokemon.
I beat the whole ruins of Alph "mystery" and I didn't discover a secret.
You must have a Secret Slab/Mystery Part. He is in Dark Crater, 10F.
concealed ruins with secret slab or mystery part
Darkrai is at Miracle Sea, but you need a Secret Slab or a Mystery Part.
the marina resort
It is in Final Maze 40F you need secret stone plate or enigma part in bag (secret slab or mystery part)
first you have to get the secret rank to do it and it will tell you from there