where sherlock doesnt know that mycroft and lestrade are going out, for whatever reason you like, and then he finds out.
bonuses-
-sherlock flips out
-john already knew
-sherlock deduces either mycroft or lestrade in front of the entire yard
-sherlock gives lestrade the "break his heart and im not the only one who will kill you and also your incompetent team will never find your pieces"
-sherlock gives lestrade tips about how to handle mycroft

ya, so thats a lot of possible things, but thanks sm