So this is an admittedly bad piece I scribbled down in ten minutes while waiting to get into the design lab. It's now two hours later and I still haven't gotten in, just in case any of you were wondering. And my project's due tomorrow. FML.
Sherlock observed things. It was what he did. He did it automatically, instinctively, and so accurately to be frightening. In fact, he did it so well he'd made a job out of it.
When he first saw Dr. John Watson, Sherlock observed that he was veteran (Afghanistan or Iraq, the question remained), had a psychosomatic limp, and an alcoholic sibling. He knew most peoples' life stories upon meeting them, and never stopped observing. Everything could be a threat, and so everything was recorded, deduced, and classified.
So when had he stopped observing John?
When John walked into the pool house, Sherlock should've immediately seen something was off. The parka he was wearing was too heavy for the weather, and didn't sit quite right, obviously concealing something bulky underneath. John's face was pale and his movements stiff, he squinted at bright lights, an effect of a head injury or drugs. He was not there of his own free will, and Sherlock should've seen that.
He didn't.
Instead he was overcome with a sense of such profound shock that he was temporarily at a loss of words. The world's only consulting detective, a genius in his own right, was absolutely baffled when faced with this one man in a place he should never have been.
John? Why would John…?
Somehow during the short time Sherlock had known him, john had become safe.
He could still observe him, and oh, he did, to the point that it had become a game between the two of them, but if in a dangerous situation John appeared it would be greeted with relief not suspicion. Here was someone to help, Sherlock trusted him.
That's saying a lot for a man who honestly considers his own brother to be an enemy.
So now, having escaped from the pool with their lives intact, all Sherlock could think about was why.
He should've trusted in John, should've known he couldn't be Moriarty. Wasn't that a kicker, Sherlock Holmes trusting somebody and feeling something resembling guilt over a moment of doubt?
Sherlock doubted everybody.
Yet all he could think about was that he should have known.
He tried to ignore the sense of relief he felt when John was revealed to be the victim rather than the mastermind. It was a little to illogical for him to handle.