A couple of weeks ago, I blogged about a Sherlock deleted scene. While Sherlock is in the hospital during "His Last Vow", he receives a single rose from a mysterious W.
I speculated on numerous possibilities as to who W could be. However, while browsing the internet, I stumbled upon a post that said the symbol by the W matched Irene Adler's wallpaper. So, the mystery has been solved.....Unless, Moffat and Gatiss are messing with us. This is something they would totally do. They could spin it as Moriarty and/or Magnussen trying to mess with Sherlock, something both characters would do.