I have been utterly obsessed with this eye for three months. Sorry, left my flash on. It's at the very end of the Sherlock ep The Blind Banker, a wonderfully apt title. Note of interest for actor spotters- Gemma Chan, who played Soo Lin Yao in this ep, now plays Anita in Humans.
This comes from a lifelong obsession with eyes in the first place, but what's bothering me is that this hint at something seems to have been dropped entirely ever since. We don't know if it was washed off, painted over, or remains all these years staring ominously at Sherlock's flat.
I dig around for other fan interpretations and explanations. This one is pretty good (just a hint of what I could be bothering you guys with). Couple of thoughts after reading that interpretation.
1- I'm irritated that Sarah never once thought to simply kick and tip her chair over after being courageous enough earlier to hit an attacker over the head. That was the worst kind of female helplessness I've seen in a long time. She was way too smart for that in every other scene. @bonenado is irritated that John couldn't hang onto her, says he likes Sarah better than Mary.
2- I subscribe to Mycroft being M here, even without having realized the soundtrack themes were supporting this. The second Shan said she appreciated the help getting into the country, I knew it had to be government.
3- The mirroring is probably why I'm obsessed with the entire series. The visual subtext about siblings speaks deeply to the horrors Mycroft finds himself caught up in. That this series could be so thorough and all-encompassing without having to drag through it with conversations explaining everything to the audience is brilliant, and I thank Moffat and staff for assuming we are intelligent. They go to great lengths, and if you like obsessing over details and editing mistakes, here is a fun wikia for you.
Anyway, the eye is a really big deal. I'm not talking about the all-seeing eye or the evil eye motif, which China doesn't have. I'm talking about being marked or tagged, a visible warning. Who did it? Shan and her people were already dead by this time. I've been a triad fan since my first Jackie Chan intro many years ago, and that eye looked more Egyptian graffiti than anything. I wound up in weird places like this and still didn't find a connection. Did Mycroft have it done? If you'd love to lose yourself obsessing over ancient symbols, knock yourself out. I was raised by a man obsessed with numerology and symbolism in prophecy, so to me this is like brain candy.
Lurkers have probably noticed I seem to avoid certain kinds of music vids. It's true I'm not into watching mushy collections of stills, especially when I'm making it obvious now that I have no interest in infatuation (asexual with prosopagnosia, sorry Benedict, love you anyway), but I have collections of music that help me think really hard. It's a sort of brain training technique. I use particular music patterns to push myself to GET TO WORK and FOCUS or to sort my moods so I won't kill people (metaphorically), and I use this mesmerizing rhythm stuff like a string to follow way deeper into my head. I don't watch them, just put my headphones on and think. iio especially is awesome for deeper thinking. But you can watch them if you want.