(I sort of imagined this post-SoT, but post-hlv is possible, too, as long as sherlock is living alone at baker street and john is with mary)
John once scoffed at the idea that marrying Mary could in any way change his relationship to Sherlock.
Fact is, he hasn't seen his best friend in over a month.
Meanwhile Sherlock is stumbling into a hospital, clearly heavily wounded and abused, demanding to talk to DI Lestrade. The nurses don't want to upset him even more and do as he says. When Lestrade arrives at Sherlocks hospitalroom, the latter tells him to call off the search for him, everything is alright, would you please phone John, he must be out of his mind by now.
Lestrade doesn't understand a word and is frankly, after a long day at work, a little on edge, so he asks 'why would someone search for you? what the hell is going on?' and a confused Sherlock replies 'But I've been gone for nearly two weeks...' only to find out, that noone fucking noticed.
+10 the reason mrs. hudson didn't ntoice is that she's on holiday or something
+100 molly and/or anderson had been the only ones to worry about sherlock
+10000 john feels reeeally guilty
+10000000 john actually thought about sherlock a LOT during that month, but he always dismissed it or felt stupid about worrying
+100000000000 mycroft didn't notice, cause prior to sherlock vanishing they had a major argument with each other, which led to mycroft cancelling all surveillance on sherlock; he, of course, feels incredibly guilty about everything too.
+ infinity for heartbroken sherlock trying to isolate himself after all this, but having nightmares and panic attacks and basically in dire need of some tlc
+ my neighbour's life if you sooomehow manage to fit johnlock in there :D