That Persian slipper floating in the water also alludes to the one used to hide Sherlock’s tobacco in the same story.
The third episode hasn’t been named yet, so maybe the final installment of series four will take its plot from that particular tale.
The sheet music floating around in the water is titled ‘Miss Me?’ and we all know where we’ve heard that before.
While Sherlock said that Moriarty is definitely dead at the end of the Christmas special, this could be a sign that Moriarty will haunt him long after.