Let's go back to that gray hole in the corner. Try using the metal rod to pick out the blue item that's in there. Hopefully the arm length combined with the rod would be enough to reach it.
Mr. Stick thinks that even his hand with metal rod can't reach object at the bottom of corner hole, since it's two-three human arms length, and metal rod's is just about 1/3 human arm length. Anyway, he decides to check. Plus, after he inserted rod inside wall hole, bottom of corner hole could lift or something like this.
Mr. Stick takes metal rod back.
*click*
This sound appears again...
And then he goes to check corner hole.

No, it seems that hole's bottom didn't move anywhere, and he still can't reach object.