Shhhh! Achievement

[IlyaALanda]

In preparation for PF release, I'm trying to get as many of CW3 achievements as possible, and I'm stumped by the Shhhh!(Find All hidden areas in the Credits mission) achievement.
I have scoured all the rooms in the Credits dungeon before picking up the trophy the ends the level, but still not getting it.
Is there an invisible room that I need to reveal somehow?
Thanks

[Karsten75]

There are two (if I recall) invisible areas. They are near other objects. So go close to the wall in the areas where you destroy something. I think, it's been a few years since I played that.

[GoodMorning]

Turning on auto fire and checking where you see explosions can be a good bulk test.

Or, once you know the layout without looking, look at the world outline, on the screen where it says "Fortress of Ultimate Darkness". Check where it does not match.

[IlyaALanda]

Thanks! Got them.
Tiny things, aren't they?

[GoodMorning]

(I resorted to the editor, and it still took a while to find. Applause Virgil.)

[balancetraveller]

Walk by the wall and check every corner ;)

[Relli]

(I wasn't gonna say anything, but since this topic got pulled back up...)
I had an experience I thought was a glitch in that game. I destroyed the emitter in the room and all Creeper, yet more kept appearing out of nowhere. After shooting at it for a bit, I had a brainwave and went in to find a secret room. The Creeper had gone in unimpeded, and was pouring back out of it like normal from beyond where I could see. I found the second room the same way, this time intentionally. If you destroy the Creeper before finding the wall, I believe someone else already mentioned using your bullets/explosions to see where they hit walls and where they fly through walls. That works as well, but is pretty slow going.