See? That smile? Sherlock stops in the middle of his showing off and goes see John. According to Lestrade’s surprised face at first, this is something that’s never happened before. Sherlock misses the opportunity to find a murderer to protect the man he just met, pretends he’s in shock and goes talk with that man. This doesn’t look like a logical reaction. It looks like a human reaction. Caused by emotions… Shock? Really??
The explanation is so blindigly obvious even Lestrade worked it out on day 1. That’s why he smiles.