Heavily implied slash- all K rated, though.
It was easy to see the thread that connected them. It wasn't visible, tangible, touchable, but it was so clearly there. It was the strangest thing Greg had ever seen, and he'd seen unbelievable things- working with Sherlock Holmes saw to that. After all, somewhere in Scotland Yard, there was an official police document beginning 'the glowing badger was a false alarm'.
But this was something altogether different. It wasn't creepy, just… enigmatic. John would walk into a room and Sherlock would greet him by name, without once turning around or looking up. Now, Greg was pretty sure 'John' was just Sherlock's default address for 'anybody not holding a gun to my head' by now, but it wasn't just that. John could do the same thing- and he knew where Sherlock was. Greg would ask John where the detective was and be told 'he's back at the flat picking up some things'. Sherlock hadn't told him that. John hadn't seen him. When Greg questioned him, John just shrugged. 'I can tell,' he'd reply.
It wasn't just location either. Greg was half convinced they had special alarms that sounded whenever the other one was sad, angry or just plain lonely. Sherlock never answered his phone at a crime scene. When he was engrossed in a body, mind tripping over millions of possibilities and narrowing them down, it was near impossible to break him out of it. But Greg remembered on seemingly standard occasion when the phone vibrated against Sherlock's leg and he'd shot back from the corpse immediately. Seemingly forgetting it entirely, he'd sighed and stated to nobody in particular 'Harry's started drinking again'. He had then pulled his phone out his phone and read nearly the exact same text out loud.
Greg would ask John why Sherlock was in a bad mood and John would reply 'Oh, he has a headache'. But the next week, when Sherlock was in seemingly the exact same bad mood, John pointed out that the way Sherlock had been standing combined with how often he checked his phone was an indicator that he was having a fight with Mycroft- probably over his safety, because if it was just Mycroft trying to persuade Sherlock to do something he wouldn't be nearly so annoyed.
Greg had put it to the test a few times, and asked Sherlock what was wrong. Whilst at first the only answer he ever got was 'you exist', eventually Sherlock would admit the truth. John was always right.
It was like a special kind of deduction that only worked for the two of them. They knew each other's footsteps, faces, every inch of the way they held themselves and every note of their speech. If a single thing was off, they could pick it up. At times it made Lestrade jealous. He loved his girlfriend, but they didn't have the incredibly connection that they seemed to. The walnut stuffed chicken he had made her once was a nice gesture, sure, but he had only remembered her nut allergy after she had taken two bites. She'd been nice about it and the paramedic had given him the Christmas hat from his cracker, but that wasn't really the point.
The thread seemed to tie around them and behave as this invisible connection everywhere that they went. It recoiled if they strayed too far, sending them pinging back together. Whenever Lestrade saw Sherlock, John was by his side. If they were ever apart for a long space of time they were visibly uneasy- fidgeting, constantly checking their phones and, in Sherlock's case, occasionally phoning Mycroft to have John helicoptered over. After the fourth time, Mycroft politely requested that Sherlock take a course in dealing with separation rather than enlisting the military every time John visited his sister. Instead, Sherlock declared that he suspected John to have epilepsy and took to accompanying him everywhere. John didn't seem to mind. In fact, Greg swore that he once overheard him faking a seizure to end a telesales call.
Sherlock would catch something John threw without looking up once. John would have brought Sherlock those things- things he needed- without the other man having to ask. It was one of the most intriguing and, shaming though it was to admit, heart-warming things that Greg had ever seen. There wasn't a breath Sherlock Holmes took that John Watson didn't know about.
So when Greg read the finishing words 'He was my best friend and I'll always believe in him', he believed them too.
And if, by some miracle, Sherlock was still out there, that thread would never be cut. One day, it would grow too tight. Greg knew that Sherlock would come hurtling back, and he knew that John would be waiting.