They're under the bridge now, trying to avoid the worst of the mud as Sherlock stalks the drug dealer who Sherlock knows to be the perpetrator of a Moriarty-orchestrated murder. Initially, John was angry at Sherlock for not telling Scotland Yard immediately when he found the evidence, and he's still not entirely happy with Sherlock even after Sherlock promised him he'd only delay reporting it for one night, regardless of the outcome of this particular hunt.
Suddenly Sherlock glances to the side and stiffens. Without a word of warning, he catches John mid-stride and crowds him up against the bridge pillar.
'Sherlock, what are you -' John starts to demand, but Sherlock shushes him with a sharp shake of his head. He leans down and breathes in John's ear, 'There's a second one right to the left of the bridge. He'll see us. Act natural.'
Before John has a chance to respond, Sherlock covers John's mouth with his lips. His right hand stays leaning against the brick wall, while his other hand sneaks down to wrap itself around John's waist in an utterly clinical re-enactment of a lover's embrace.
John is torn between laughing at the utter absurdity of it all and berating Sherlock for once again ignoring anything resembling social mores or personal boundaries. He settles for grabbing Sherlock's scarf in a (probably futile) effort to stay at least somewhat in control of the situation.
A decidedly female gasp of surprise causes him to look up. He catches sight of Sarah's shocked, vulnerable face before she turns and walks away quickly.
'Sarah, wait, it's not like that -' he calls after her feebly, but it's already too late as her dark red coat vanishes in the darkness.
'She was following you, spying on you; you wouldn't have lasted much longer anyway, and the marks on her -'
John cuts him off. 'Don't. Just - don't.' He's so furious he can hardly speak. He pushes Sherlock off him with rather more force than necessary (but, he thinks, entirely less than the situation warrants) and storms off after the drug dealer.
After a moment's of hesitation, Sherlock follows.