The third secret spot is a stack of two hidden walls in the left face of the gold brick wall. The top one (which you are showing) is empty. The healing item is in the bottom one.
The natural place to attack from is the small floating block to the left. Attacking from there hits only the top wall. After 5 hits it opens and gives nothing.
After that, nothing reaches the bottom wall: not a crouching attack on the block, not dropping off it, not jumping and attacking from the floor below. A few attacks while falling past the stack did hit the bottom wall once each.
From code I can see the item being there