Page "The Adventure of the Empty House" Paragraph 16

Holmes is expecting an attempt on his life that very night, well aware as he is that Moriarty's men know that he is back in London.

The police, unknown to Watson at this time, are nearby, having been told that they will be needed.

As usual, Holmes has deduced everything correctly, but with one almost disastrous exception: he fails to anticipate that the would-be murderer might actually use the same empty house for his nefarious deed that he and Watson are now using as their vantage point.

In he comes, with his specially designed airgun, utterly unaware that his intended victim is right in the same room because it is dark.