That part, he wasn't expecting.

His first thought is How?, but his ruthless imagination answers it without a pause. Of course that Obadiah would not have done it on purpose—he is, bless him or damn him as you like, too smart for that. It would have been an accident. One he was given cause to regret, Sherlock expects. God knows you would not need to remove his soul to put him in a vicious mood under such circumstances.

All this flashes through his mind in hardly a breath; disinclined to reveal any of it, he voices only a very subdued, "Oh."