Despite what people seem to think Sherlock has a very clear concept of personal boundaries. He often ignores them, mind, but he is aware. And for a heterosexual, British male, John's boundaries are surprisingly flexible. John will make allowances for Sherlock. Apparently, simply on the basis that he is Sherlock.
Sherlock regularly takes shameless advantage of that fact.
But even between them there are lines; things which are neither negotiable nor remotely close to flexible. John, for instance, steers clear of Sherlock's violin. If he absolutely must move it (generally to clean) John will pick it up with incredible care and move it no more than a few inches. He shows no such restraint with Sherlock's experiments, nor the body parts that tend to accumulate in the fridge, yet the violin is sacrosanct. It's a bit unnecessarily pedantic, but Sherlock appreciates the distinction just the same.
Sherlock, for his part, never, ever touches something that belonged to Mary. There aren't many of these items – John is a practical sort and kept only a few mementos, and most of those stay in his room – but what little exists is strictly off-limits. Not because John has ever said as much, but because even Sherlock high-functioning-sociopath Holmes can recognize practical limitations. Mary's things are so obviously crossing the line that Sherlock doubts he'd still have a flatmate if he went there.
There is only one exception. A bound leather frame (antique, obviously inherited) with a picture of the late Mary Watson-nee-Morstan grinning at the camera. Sherlock is remarkably fond of this photo. Because Mary, very clearly looking at John and just as clearly in love, is posing with the skull. His skull. The one that even now has pride of place on the mantel.
It makes the picture, in some inexplicable, sentimental way, Sherlock's.
It is beyond strange that some part of Sherlock wants to claim a piece of this woman. But she has claimed a piece of John, a piece forever more stamped with the name Mary, and dead or no that is not changing. It means something. She means something, this woman that he knows through deduction and through John but whom he can never meet.
She had made John happy.
Sherlock even begrudgingly suspects he would have liked her if they'd had the chance to meet. If she had been a client she would be the type to come armed with everything Sherlock could possibly need. She would be the type to pass him an interesting case without being boring or annoying. She would, Sherlock thinks, have been that same type of intelligent that John and Lestrade and Mrs. Hudson are. Not a genius, but also not stupid. At least not all of the time.
He talks to the picture sometimes.
Invariably he talks to her about John. Always with the same words ("Your husband is being dull today...", "Tell your husband to get my mobile..." , "What are you looking at me for, he's your husband..." and once, only once, "I'll take care of your husband for you.").
John has caught him at it once or twice. He always appears bemused by the concept, but there is something else there too. Something fond and almost grateful. So far, John has not ventured to comment on Sherlock's new habit.
John talks to her too, though only when he thinks Sherlock can't hear. John tells stories, remarks on his day, talks about the weather, and he tells her about them. About John and Sherlock and their cases. Once, he apologized for being happy and Sherlock had to leave the flat or risk John seeing his expression.
But he plays for her that night. Picks up his violin, makes a few quick deductions about the music she would have liked, and plays.
John hears him and knows and it's just a picture, but Sherlock does it anyway.
The next day John sits him down and hands him a string of pearls and tells him about Mary. About meeting her, and loving her, and everything she had done for him while Sherlock was... was not there.
Sherlock is left uncharacteristically speechless.
And John smiles at him and says, in a perfectly serious tone, "It's alright. She would have liked you too."
Sherlock looks at the picture, looks at Mary and his skull, both grinning in perpetuity, smiles and silently makes his promise one more time.
I promise you, Mary. I'll take care of our John.
And Mary grins back.