He almost chuckles as he notices the passerbys begin to queue up, emboldened by what they had just seen. No one likes to be the first to do anything. Especially not when what they want to do is shooting them annoyed glares. Seeing Sherlock kiss him made them realize that the detective was willing to play by the rules no matter how much he hated it.
For a moment Sebastian considers sticking around. The boss would get a kick out of the story of Sherlock's discomfort. Before he could find a comfortable spot to settle for awhile, however, the detective comes hurtling over the table yelling something about an emergency.
Sebastian sighs as he catches the other's eye. "I better not spend the night in prison for this." He huffs before pulling out his gun and firing, hitting one of the passerbys in the leg. The shot wouldn't kill him but it would hurt and he would need medical attention. It also served to gain a bit of attention. Sending people scrambling as the police officers stationed nearby struggled to made their way towards the sound.
"Come along detective, catch me if you can!" He called out before taking off down the street. He just had to get them far enough away so he could return home and Sherlock could escape his charity event without raising suspicion.