This is the context from "Sherlock season 1 episode 1"

Sherlock(to Watson): "Potential flatmates should know the worst about each other "

Watson(to Mike): "do you told him(him = Sherlock)?"

Mike(to Watson): "Not a word." 

Watson: "Then who said anything about flatmates"

Sherlock(to Watson): "I did. Told Mike this morning that I must be a difficult man to find a flatmate for. Now here he is just after lunch with an old friend just home from military service in Afghanistan. Wasn't a difficult leap."

Watson(to Sherlock): "How did you know about Afghanistan?"