I'm sure Lestrade offered to help, thinking it was just two drunken blokes, and then got the biggest surprise of his life. He saw it was John and Sherlock and turned straight around like "NOPE. Not my division.
THAT was the actual moment I started shipping Johnlock. John loves him, and gets super jealous when Irene flirts with him. I still ship Sherlolly harder, but of Johnlock happens I will be perfectly fine with that. <--- I prefer Johnlock.