Hiding in plain sight, I think either Gatiss or Moffat mentioned something about that, and the idea has interested me a lot. Lots of people say, 'if you want to hide a diamond, hide it in a room full of diamonds'. Well there is also another way of hiding things (obviously, there are tons of other ways to hide things) which is much more to the point of hiding in plain sight. Hiding the object in a place where it's supposed to be. Let's say you want to hide a necklace, you put it in a jewlery display; you would see it, your eyes would pass right over it, but that's the thing, your eyes don't focus on it, you wouldn't acknowledge it because it is 'supposed to be there', or rather, you expect it to be there. Of course, there is always the, have a room full of diamonds, people would check each and every diamond, but that particular diamond is hidden somewhere else. 

Anyway, as for the "it's something Sherlock did that was very out of character" clue, I came up with a whole list of things that Sherlock did that were uncharacteristic, and in all honesty, I think Moffat just said that to make us, fans go even more crazier than we already are. It could be a clue, but I have my doubts. 

It's really annoying, because we've been informed that "all the clues needed to solve TRF are in the episode" and here I am, with no idea (well, I have plenty of ideas but none of them fit all the facts) how he survived