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When John is speaking to Sherlock's grave, we can see Mrs Hudson walk away in the background. When she passes a tombstone, an arm is seen that seems to pull her behind the stone.
When John leaves the graveyard, Mrs Hudson is nowhere to be seen.
Who was the person behind the tombstone? Could it have been Sherlock? Has something happened to her or is she simply waiting for John outside the graveyard?
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Yes, I noticed that ages ago and came to the conclusion that it is of no significance whatsoever. Maybe a member of the film crew was simply dragging her out of camera's sight.
Mrs Hudson would wait for John until he followed her. Probably the cab that had taken them to the graveyard was still waiting for them to return.