Looks like a Bug -

In the first chapter; Amazing Larry, he has a mystery item in his own puzzle (the Joker)

In the third chapter, the question mark appears again on his own puzzle, which is now entered via the 3-letter word-square. However, no mystery item appears either in the word-square entry or the puzzle itself, where the joker had been in chapter one.

Unless of course, I have missed something.

is it showing on the section for his word puzzle or the magician area itself? If it is the latter, try looking at the padlock on the water escape cage.
Hope that helps

It shows up on his word puzzle in the SECOND chapter. The question mark appears until you enter the puzzle, then vanishes after you exit.

The morphing padlock was in the magician area, in the FIRST chapter.