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With Alfredo gone, the house is quiet: no more Sherlock quizzing him about the proper way to hot-wire cars made in Japan between 1995 and 1997; no more abrupt blasts of noise from the car alarm as Sherlock learns his way around the system. The sudden lull is almost as jarring as the noise had been, and so Joan gets up from the sofa and goes to make tea.
She brings a mug out to Sherlock when it's brewed (steeped for three minutes, Watson, no more and no less, no sugar, never Lipton's) and finds him frowning at a chemistry text with apparently ferocious concentration. Joan holds out the mug to him until the bitter-rich smell catches his attention.
"So," she says when he looks up, "Spongebob Squarepants, huh?"
Joan thinks she knows Sherlock well enough now to be able to parse his thought process through the expressions that flicker across his face: the moment where his interest is caught by something he wasn't expecting; the brief loss of focus as he mentally cross-references his index for Watson, Joan with that for a children's TV show; the way his eyes widen ever so slightly as he makes the connection.
"Well, Watson," he says as he takes the mug from her. "Children are, ah… cartoons are rich with cultural cues that are needed if one wishes to adapt to certain cultural, uh…" His fingers tap in a syncopated rhythm against the ceramic; his gaze darts from the wall to her face to a point just over Joan's left shoulder. "I'm going now."
Sherlock bolts up the stairs and Joan represses a grin. She's seen him interact with suspects, knows he can fake accents and identities with aplomb—Sherlock Holmes has a poker face that doesn't fail him when it comes to hardened killers, but which crumbles when she teases him a little for watching kids' TV. Maybe Joan doesn't know him half as well as she should, but what she does know of him—well, she doesn't think she'll ever be half as scared of Sherlock as he seems to be of himself.
Several days later, Joan's in the hole-in-the-wall thrift shop three blocks down, rummaging through the dollar pile. She picks up a cherry red scarf that looks perfect for fall in New York, and beneath it spies an adult-sized Spongebob Squarepants t-shirt. She smiles to herself, hands over two dollars to the woman behind the counter, and leaves the t-shirt on Sherlock's bed when she gets home that evening.
He never mentions it to her, but two Saturdays later he comes bounding down the stairs at seven in the morning, yelling, "A case, Watson, a case!", a muddy green cardigan buttoned haphazardly over Spongebob's smiling face.
Gregson looks questioningly at her when they show up at the crime scene, but Joan just shrugs. "Adapting to cultural cues," she says, as if that explains anything.
From the quirk of Gregson's eyebrows, maybe it does.