Spies
It worked perfectly at university. Sherlock's first roommate would have moved out after less than a week if not for the handsome remuneration from Mycroft for some harmless prying and snitching. As it was, he lasted the whole term. The next roommate lasted nearly a term, the third barely a month. Sometimes Mycroft wondered if Sherlock knew and was driving them away on purpose, was making himself out to be even more inhuman than he was just to make his brother worry. Still, they were successful, with their watching and snooping. That was how he found out about the cocaine.