Sherlock: Unlikely he would notate trips to underground Chinese gambling parlor. Bell: Excuse me? Sherlock: That's what he was doing last night. Bell: I know you're just waiting for me to ask you why you think that. Sherlock: [pauses] Bell: [sigh] Why do you think that? Sherlock: I'm glad you asked. It was the 13's. He apartment number 13 His cell ends in 1313. He chose this office 13.