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Proprioception: (n) /prō-prē-ō-sep-shən/
the ability to sense the position, location, orientation, movement, equilibrium, and condition of the body and its parts without relying on visual input. (From the Latin proprius, meaning individual or one's own, and perceptio, to perceive, observe, know, or understand.)
John always knew where Sherlock was. In the same way that he always knew where his feet were, or his hands: not necessary to consciously think about it, but the knowledge was there when it was needed, swift and sure and instinctual. He might not always have eyes on Sherlock, but apparently his brain had decided to keep constant tabs on his precise location.
it was Mrs. Hudson, of course, who noticed it first.
"It's funny watching you work around him, isn't it?" she said with a gentle smile. She had come up to their flat for dinner, which John was assembling in the kitchen while Sherlock dashed about behind him finishing up something best left unexamined with somebody's donated liver.
John thought about it as he wove seamlessly through Sherlock's frenzy, never colliding, never an awkward mis-step, turning in tandem with his madly rushing partner. The experiment was concluded and cleared, and dinner on the freshly-disinfected table, within the space of half an hour.
"It's a defense mechanism," John told her with a twinkle as they ate. "Before I developed the skill, I'd have been more than likely to turn and get a face-full of liver as he crossed behind me." Sherlock sniffed his disdain, as if he would never allow such a disruption to his results.
But now that it had been brought to John's attention, he noticed it more and more often; unless they were at opposite ends of the city, he could always tell exactly where Sherlock was.
In the flat, there were many tells to excuse his newfound super-ability: the floors creaked, fabric rustled, the violin screeched or sang, glassware clinked on the experiment table, the laptop keys clattered.
But he noticed it even in less-familiar environments. At crime scenes. In Bart's lab. In a crowd, surrounded by sixty other passengers on the tube, he could have turned, blindfolded, and pointed unerringly to Sherlock. The one time Sherlock came into the surgery unannounced, John paused in the middle of a sentence and turned as though he had heard someone call his name, already knowing who he would see walking through the door.
It was not just location, either: as John started to test his theory, he noticed that he could almost always predict what position Sherlock would be in when he turned, what his posture would be, if he were holding anything in his hands, where his attention was directed.
It was Lestrade who pointed out that it went both ways.
"Did you know he's mirroring you?" the D.I. whispered in passing at the end of a case, as Sherlock did his rapid-fire wrap-up speech for them. The case had been convoluted, bloody, and disturbing, and had made John extremely uncomfortable throughout. He was standing in parade rest, as he had a tendency to do when feeling threatened -- and so was Sherlock, for whom the attitude was not habitual.
So John, needing a distraction from the recitation of all the unsavoury details he already knew, borrowed a leaf from Sherlock's book and quietly began an experiment: John crossed his arms.
Half a minute later, after a grand illustrative gesture, so did Sherlock. The unaccustomed strain of his shoulders ruined the lines of his bespoke jacket.
John crossed the room to stand by the last victim's mother.
Sherlock did not move, but he shifted minutely so that he was always facing a little to one side of John, never letting John out of his peripheral vision, shoulders angled towards him.
It was like watching a compass needle reorient on North, and John realized Lestrade was right: Sherlock was magnetized to him, too.
Later that night, as Sherlock handed him a (perfectly doctored) mug of tea, he said, "You always know exactly where I am in any given room, don't you?"
Sherlock gave a start of a frown and answered, "Yes?" Not as if he were uncertain, but as if he were confused that John thought it a revelation, as if he had always been aware and were a little surprised that it had taken John this long to notice. Which was probably true.
Now that John was mindful of it, he began to take advantage. When Sherlock asked him to pass something small and relatively unbreakable, he would toss it over his shoulder without looking up from his book, and Sherlock never missed the catch. When running down an armed suspect in a multi-level warehouse, John allowed himself to deviate from his planned route, noticing a distinct tactical advantage in a higher position previously unseen by either of them, knowing Sherlock would understand and anticipate what he had done and react accordingly. When John delivered an opinion at a crime scene and Sherlock tilted his head just a fraction, John felt the ripple in the electro-magnetic field and re-evaluated, knowing he had missed something. When Sherlock's sniping crossed the line from merely abrasive to deeply wounding, John only had to shift from one foot to the other to signal a verbal retreat, and Sherlock subsided.
Very quickly John came to realize that their awareness of each other extended beyond simple location and orientation into the realm of emotional state and physical discomfort. Each could tell, almost before it manifested, when the other was compromised.
Two days before John's week-long incapacitation with the flu, Sherlock came home with six boxes of tissues and a fresh supply of paracetamol. When John found his neck prickling as Sherlock's spiraling temper plucked at his attention, he found he could head off an incipient black mood if he texted Lestrade to beg for cold-case files a day early. If Sherlock knew John had been particularly unsettled by a case, or by a bad day at the surgery, he made tea for him without being asked, and let himself be bullied into eating and sleeping with only a token protest. And when that had not happened in far too long, when Sherlock suddenly stopped his frenetic examination of an ongoing case in the middle of a word, John stepped up to catch him before his eyelids had the chance to flutter or his knees to go limp, before anyone else (including Sherlock) had realized he was even close to collapse.
It was a bit like having an extra limb: you always knew what it was doing even when you couldn't see it, and when it was injured or fell prey to pins and needles through inactivity, you instantly knew. The fact that John's extra limb was usually across the room and shaped like a mad genius was irrelevant.
This had a bit of a downside: physical injury to one half of the organism often led to impairment of the other. When John (whose sixth sense apparently did not extend to desperate embezzler's wives armed with cast-iron pans) was hit over the head, he regained consciousness to find Sherlock cradling his tender skull in swollen hands whose knuckles were stained with blood, and Lestrade had to take both husband and wife to A&E after arresting them. That incident earned them a stern lecture from the D.I. on the related subjects of waiting for backup (which John promised to do) and the use of unnecessary force (for which Sherlock refused to apologize, and John strongly suspected Mycroft had something to do with the fact that the incident was never mentioned again.) When a dangerous serial rapist's bullet broke Sherlock's humerus, even though the detective insisted he was fine and that the greater good was at stake and the public still at risk, John flatly refused to continue the chase alone, threatening to sell Sherlock's violin to a pawn shop unless he sat down to wait for the ambulance. Since their quarry ended up in a bloodless shootout with the Met Police only two blocks away before falling fatally into the Thames, John could not bring himself to care much, although Sherlock grumbled about missed chances and mistaken priorities for weeks.
Sometimes John felt it was like gravity, much as he knew Sherlock would have despised the astrophysical metaphor as both overly poetic and deliberately insulting, given the Yard's reaction to his knowledge of the solar system. Also, John knew it wasn't entirely accurate, given that he couldn't decide who exerted the greater pull. Sometimes, squinting in the face of Sherlock's overwhelming and self-destructive brilliance, he felt like an insignificant planetoid orbiting a blue star, circling helplessly and unable to leave as his world was overshadowed by the light of an impending nova. Sometimes, as Sherlock's undeniable tug drew him beyond the event horizon of a dark alleyway where anything might be lurking, it was like being attracted to a black hole.
At other times, he felt like Sherlock was his satellite: a fiery-tailed comet, caught out of the midst of some some unimaginable journey by John's modest gravitational well, come to observe his homely little planet and delight in all the life-forms contained therein. At times, Sherlock was the silver moon that lit up the darkest of his nights, reflecting forgotten sunlight back to him when he was lost in the shadows. He sang in the tides of John's blood, stimulating and grounding at the same time, his mere presence urging John to new highs, making his seas surge and his tectonic plates tremble. In social situations, John was his sun, stable and comforting, and Sherlock was an uncertain extra planetary body, not quite on the ecliptic, who hovered around John's warmth, dodging between the other planets but interacting only with him, soaking up his affection and baffling the astronomers who attempted to chart his erratic progress across their skies.
But when they could simply be alone together, companionable and quiet in the warmth of their flat, it felt like two asteroids orbiting around each other, or a binary star with no satellites. Lonely little unstable celestial bodies who had found their perfect partner, each with their own agenda but inseperably linked, circling round and round in an uncaring universe, attracted only to each other, needing nothing else. A complete system unto themselves. Each always aware, by the magnetic pull of the other, of exactly where he was.