Before answering Sally's call to the door, Mary drew her authority as a teacher, as the mistress of this house and as a learned, cultured woman around her like a cloak. She assumed what her brother had always referred to as her "nobbish" air - but he had done that mostly because it made her uncomfortable. It was a particularly balanced thing: to behave above one's self was patently ridiculous. One was best off simply knowing the full and absolute bounds of one's own authority, dignity and grace.
Which she did, thank you very much. And so, invisible cloak assumed, she joined the Inspector where she had told Sally to usher him, in the (currently spotless) parlour.
She let Lestrade clatter to his feet in her wide-eyed, gently feminine silence, her entire being an innocent question about his presence while allowing her own to get right into his head and confuse him. Then she graciously asked him to sit, and was seated herself.
Sally, who really was getting very good at anticipating these things, bless her, brought the Inspector a cup of strong tea with milk and sugar. A nice touch, not asking, because now he could not refuse hospitality and that would put him a little bit in Mary's debt. She took her own, lighter tea from Sally with a smile and asked Lestrade what she could do for him.
It took all of her not inconsiderable self-control not to snort with very unladylike giggles a few times during Lestrade's version of the current disastrous case. She felt, as she usually did, a profound sympathy for the rat-faced little man. She suspected that in different circumstances, he might actually be happy enough to let Sherlock and John just have their heads and run about the city solving large problems for him - but it was not to be, with the attentions of this person and that person of note and rank breathing down his neck. London was of great concern to them. London, after all, had only fifty years ago still had an uncomfortable tendency to turn into a mob.
But Mary listened with wide-eyes and innocent, but interested face as Lestrade continually caught himself just on the verge of using language Completely Unsuitable for Ladies, which Mary cheerfully filled in, in her own head.
"Well that sounds dreadful," she remarked. "I'm so terribly sorry you have such cause for frustration, Inspector, I truly am."
Lestrade promptly averred that it was quite all right, no trouble, only he really did need to know -
Mary sighed. "I'm terribly sorry, Inspector, but I really couldn't tell you where my husband or Mr Holmes is, at the moment. I would, of course, let you know the first possible moment, and please, do let me know if there's anything else I could do to help. No," she added, "please, finish your tea, it must be dreadfully cold out there and I would hate to think you came all the way here without some satisfaction - oh," and here she widened her eyes in pure horrified wifeliness, "you wouldn't want to search the house, would you? Would that be necessary?"
Lestrade found himself caught between trying to choke down his tea as fast as possible and assure the poor fragile creature she was impersonating that the constabulary would never dream of intruding upon her in that way.
Eventually, Sally let him out. Mary had her lock the door, and watched until she was quite sure that the Inspector's carriage had gone on its merry way, before she mounted the stairs once more, tea still in hand, though her saucer was abandoned in the parlour.
John was just finishing up sewing closed the cut behind Sherlock's ear, his scissors between his teeth.
"You are a terrible woman, Mrs Watson," Sherlock informed her, as she came over to look.
"Yes," she agreed absently, "it's absolutely dreadful, how I let you get blood all over my sheets."
John pointed to the canvas he had set out and on which, admittedly, they were sitting, with an aggrieved expression on his face. Mary blew him a small kiss in acknowledgement.
"I didn't even lie," she replied, aloof. "I only told him I couldn't tell him where you were."
"May many other things remain as reliable as Lestrade's slow wit," Sherlock agreed. Over his head, John rolled his eyes, and Mary hid a smile.