It's nothing, really. Just a dropped wallet, an excuse for Sherlock to avoid paying yet another cab fare. He stoops to retrieve it from the gutter outside New Scotland Yard headquarters, muttering the inevitable: "John, will you get that?"
John will not, in fact, be getting that. He grabs the fallen wallet before Sherlock can, passing it to him, his thumb brushing against Sherlock's fingers. They tremble like fire, the blood thrumming under his skin. And John remembers the glass-strewn sitting room back at 221b; the violin held, not played.
Perhaps, he thinks, not all hand tremors are psychosomatic.