This was taken from insanedifficulty.com:
Chest 13: Flower
If you enter the bottom right trunk pipe in the area with 7 trunk pipes, then walk to the bottom corner of the cave, the chest is there in the very middle of the lightened circle near that bottom corner.
I found that area mentioned, but there was no hidden chest there. What does the info refer to?
Edit: It was there. Apparently that trunk send me to a different kind of cave, instead of the normal one.