I've found one, so far. I don't know what the floor was called, but it's the one with a lot of grass everywhere and boxes floating off to the sides. Shooting the boxes, I found a switch which spawned a key and a few enemies. The key opened a door to some treasure that would normally be missed.
I've also noticed a weird little arrow symbol made out of breakable grass blocks in some levels, and wondered if it led to anything. If it does, I haven't been able to figure it out.
Are there any other secrets like this?
While running fearing for your life and sanity in graveyards keep an eye out. Some of the rooms with an energy door have a secret passage behind the door that leads to a few treasure boxes. It is hard to see on the screen, but the minimap gives it away as you can see a smallish square jutting out behind energy room.