In the spirit of the special, this is the full list of mirrors and heartrooms in every episode of sherlock. i posted one ages ago on an old sideblog but it was totally incomplete as i was still figuring shit out. :))
They mark mirrors with: callbacks & narrative mirroring, onscreen mirroring (interposing mirrors directly over their characters etc), sharing physical traits & clothing, sharing names, soundtrack cues, just putting them in mirrors, mirrors freaking everywhere. etc.