Why didn't Lord Coward spot the origin of the sound while Sherlock was hiding in the smoke?

The main reason is that he had not spotted that the open window gave Holmes an escape route and was therefore not in any kind of rush.

Voices tend to echo slightly in rooms and the smoke was disorientating. Too many gun shots would have raised attention which might cause issues so he wanted to wait until he was sure he had a clean shot. It was only a matter of time before the smoke dispersed, so why not wait?

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