OK... there was a fill a couple weeks ago, you may have already read it but it can be found here: iguana-dog dot livejournal dot com/2225 dot html. "Sherlock has a meltdown while at Johns and Mary wedding because of all the sensory input. Mary is the first to notice and takes him to a spare room to calm down, John soon joining them." Fic includes stimming.

For a fic more like the prompt where John sees Sherlock stimming for the first time, on AO3 there is Overwhelmed by sherlocksavant (thecaffeinatedaspie).

Enraptured by Goldenheartedrose is similar in nature but the situation and reactions are different.