Sherlock was fond of letting Lestrade, and anyone within earshot, know that he was an idiot. It usually tied seamlessly into a one way conversation about how everyone is stupid and it is unbelievable that people manage to get around their day to day lives with such a lack of brain power.
Be that as it may, it didn't take long for Lestrade to figure out exactly who had killed the cabbie serial killer. From the first description Sherlock had provided to the widening of the younger man's eyes as they settled on the doctor who Greg had only seen for the first time that day, he'd only be an idiot if he ignored these signs. If you knew the consulting detective it wasn't hard to put two and two together and being a DI with his many years of experience didn't go astray either.
Keeping all this in mind Greg was never more thankful for his choice to keep his discovery to himself as they stood and fired at the Baskerville Hound. It was there the greying man saw, and appreciated, the aim of a trained solider.
An hour of so later, coming down from the fear drug, he felt that the pint he bought John back at the Cross Keys was the most well earned drink in recent history. He knew then that this particular secret was one worth keeping for the greater good, even if it did go against some of the things he stood for. Sometimes safety was more important than abiding by the rules.