Here are some secret passages I found, there's more of course and if you find them please share them with us:
This one is in Industrial Complex 8, second floor to the bottom left of the mini-map you'll see staircase that will lead you to a room, To the top right corener of the room you will find this secret passage:
It's treasure is "Cursed Sword +4" (199 +4 ATK), it's armament drive ability is: damage dealt x3 when no shard is equipped.
These secret passages are in Salvabot (in post game):
First room:
I don't remember the treasure in these but if someone found them please mention them here.