[Of course Mycroft knows - that he asks about it is because he's interested. Now comes the part of the conversation where Lestrade is far too modest about catching a serial killer - a serial killer - and Mycroft gets to deduce.]

'Nothing much' ... your modesty really is overwhelming. From what I saw of you on the news, and the circles under your eyes, your team and yourself were working around the clock. [One corner lifts again.] And details never bore me. [He puts a persuasive purr on his voice.] Please, Gregory ... brag for me.