Sherlock sometimes uses a memory technique that he calls a "Mind Palace." This is not an invention of the screenwriters; rather, it is a method of aiding memory that dates back to Ancient Rome called the "Method of Loci". One of history's most famous real-life practitioners was the sixteenth-century Italian Jesuit missionary Matteo Ricci, who introduced the method to China.
I'm not your housekeeper.
In the American (PBS Masterpiece Mystery) broadcasts of this series, certain letters in the closing titles are highlighted red. These letters spell out a word that somehow relates to the episode. These highlighted letters don't occur in the closing credits of the original airings of the series (on BBC) though.