They're in a smoky, stuffy, underground jazz club. Mycroft's asked Sherlock to look into a potentially huge counterfeiting and money-laundering outfit apparently comprised of bored housewives, and the details have proved too interesting for the consulting detective to ignore.
John's sitting at the bar, nursing a glass of scotch, while Sherlock does his thing. He glowers bitterly as Sherlock saunters up to a woman, all toothy fake grin and tousled hair. She's smitten with him, they always are. He's probably got her singing like a canary, giving him way more information than he needs. John swigs his drink as Sherlock bores of the first woman, sidling up to her supposed partner-in-crime.
Scowling, John realises how extraneous he is right now. Why did Sherlock even insist on his tagging along? These two ladies can't pose that much of a threat, and John feels completely useless. It's almost like Sherlock is trying to make John jealous.
Sherlock's his friend, his flatmate, his colleague. There's nothing more to it than that, John keeps reminding himself, and there never will be. No reason to be envious of Sherlock shamming it up, flirting and strutting with these middle-aged tarts who fancy themselves hip and intellectual. John digs his heels into the ground, ignoring his heartbeat, thrumming in time to the music, pounding out a heady bossa-nova.