I can't see a smiley face. I think it can just be adhesive residue or yellowing after a picture or sticker is removed.
But then, it is blurry.
I would think Sherlock would need to see it if it was a clue. But it is behind the opened door and stays there. IMO, the clues are there to be seen, hidden in plain sight.
Never mind. This overtinking and imagining is what we do. If we see the sky with clouds we see sheep or a face.... (I am reading a book about perception psychology.) The human brain works like this.