Hey, I've been trying to get the completionist achievement, and am 99% done, but have hit a snag. I was getting the second last piece, a gold brick (which unlocked that secret room), but when I did, for some reason the very last piece, which was marked by a question mark vanished. It's the last armor upgrade, and I have no idea where it is. I'm not sure if this information makes any difference, but I'm at the part in the campaign right before the giant explosion in the room with the circular thing. Also, I have found all the parts of the suit since I have all the pass keys. Any help would be very much appreciated.

You might wanna check out this Shadow Complex guide, it is very helpful

I think it's in a secret spot in one of the safe rooms, because I remember missing an armor upgrade my fist time through.

Vinny posted these helpful pictures to help with a hard to find armor pack or this amazing interactive map was extremely helpful for me as well as the aformentioned guide is great.

Actually, I finally managed to find it. It was the armor upgrade in the same room as a save room, which meant that it wasn't marked on the screen. New problem though: It says I have 100% completion, but I haven't earned the achievement yet.

Beat the game and it unlocks at the final score screen I believe

Just go finish it again. It's a breeze with the helmet.

" @ProjektGill: Dang, good link dude.  
 
@piratemonkey12345: You have to beat the game with 100%, not just get all the items. I'm assuming that's why you haven't gotten the achievement.     "