"What is it this time, Mycroft?" John Watson finally said, drumming his fingers against his thigh and sounding annoyed.
His brother's influence, Mycroft was quite sure, and it almost made him smile.
Mycroft knew this was a conversation that has long been coming, for all he had hoped it would never be necessary. But.
There is nothing specific that increased the urgency of this. Nothing beyond an unease at seeing a familiar twitch to Sherlock's fingers in a CCTV feed; seeing the slightest, barest hint of a pause when Sherlock was interacting with a certain member of his network of homeless before he stalked off.
Unease was not specific enough to be going on with most of the time, but it was enough that Mycroft felt this conversation, private between him and Dr Watson, which was why even Anthea was not there and they were in an area more secure than Watson could begin to imagine, finally needed to be held. Nothing specific, but enough to create a worry that made a nuisance of itself.
Mycroft closed his eyes, for only a moment. It is the only moment of weakness he will allow. "There is something I must ask of you, Dr Watson."
John made an annoyed noise. "We have been through this. I am not taking your money and I'm not spying on Sherlock for you."
"In this one instance, I must most strenuously request that you do," Mycroft said softly, and something must have slipped into his voice, judging by the way John jerked his head and looked at him sharply. "I only ask..." Mycroft began, then stopped.
A delicate thing, this.
Most of the time, he believed Sherlock. Especially since his own surveillance had found nothing. But Sherlock was very, very good at hiding things, and hiding them exceptionally well from those he knew were looking but not watching.
John Watson, most of the time, did neither, because under the vast majority of circumstances, there was nothing he needed to or was uniquely qualified to truly see. But there were circumstances under which John was most uniquely qualified, and that was why he was the only one Mycroft knew, beyond himself, who could.
He and Watson shared only two important commonalities. One was Sherlock.
"I do not believe he is using," Mycroft said, and his voice was smaller than he would have liked. "But the...concern," he said, pausing to search for a word, "is always there."
Unbidden, an image came to Mycroft's mind, of Sherlock going through withdrawal, and the way his lip, of all things for Mycroft to remember, had trembled. Sherlock had often had the shakes as he was getting the cocaine out of his system, but it had been that, the way his lower lip had trembled, like it had when Sherlock was a small boy on the verge of tears, that had cut Mycroft to the quick and still did.
Watson had seemed torn between getting angry on Sherlock's behalf and...and between their other point of commonality, and it was that which won. Mycroft knew it by the way John's shoulders slumped. Mycroft could only imagine what Watson had seen on his face, but could gather, from the way the anger had suddenly bled away, and John's eyebrows drew together and his eyes trailed unseeing along the ground, that it was something very much like the slightly pained expression of remembrance John wore on his face now.
A single other point of commonality, of watching a sibling slowly killing themselves with their addictions, and knowing all too well the way it ripped all bonds to shreds, leaving only aching tatters too raw to heal on both sides.
It was a terrible and strange simpatico that they shared; one that could not be put into words and that could not and would not be mentioned even obliquely.
John saw, but did not watch. Or so Sherlock thought. Mycroft knew better. While it may have been true for most things, on this, John would know the tells as surely as Mycroft did because he had seen them all the same way. Mycroft had always known, just at a glance, when Sherlock had been using, even when his staff insisted Sherlock had not.
They were always wrong. Siblings knew.
"You understand, then," Mycroft said, looking at the walls. "I...I need not to be told, if you do not wish to. I will not be a wedge in this. You need not report anything to me, unless you think... Only...I simply need," he tried, and then found himself oddly at a loss for words and unsure as to how to finish.
He didn't need to.
"He is clean," John said. "But...if he starts to...if it looks like...I'll try to keep him off it."
There was no tiresome entreaty to trust Sherlock, no pithy exclamations or angry, offended accusations. Even if John had rock-solid faith that Sherlock was clean...he and Mycroft understood each other, and both of them understood addicts. He understood why Mycroft could not simply hope or believe, not fully, and how that needling uncertainty was always there; always an unbridgeable gap, the most painful and aching shredded bond, between him and Sherlock.
And John also knew how easy it was for someone to fall off, even when they were determined not to. And what it was like to watch helplessly as it happened.
Mycroft closed his eyes. "Thank you, John," he said. "I'll have a car send you home, then," he said, his eyes open and voice normal again, and John shook his head.
"I'll make my own way back. Need some air," John said dully, and Mycroft knew, again, that in this one, singular case, they understood each other better than anyone else they knew ever could.