UPDATE: I went through my item guide and decided to back clear any area where I had handwritten a notation on the guide.
Crimson Forest: Near Swamp was first. I had written a note saying "Pillar", since I had rammed a pillar and wasn't sure which one to cross off yet. Apparently I had decided that I had found everything and marked that area as clear. Went back to the "Near Swamp" area. It was all the way north west, just out of camera view. Had to pan the camera left to find it.
I know odds are slim anyone else made my mistake too, but hope it helps. Achievement Unlocked