I read a story from Masaad Ayoob where he was asked to actually open carry for a day just to see how often it was noticed. The only time he was busted was by a youngster who was eye-to-belt-high and thus stared right at it while he was in a grocery store checkout line or something similar. She busted him pretty good, and tugged on her daddy and "out'ed" him. He badged her and her dad, and dad set the young girl at ease.
So dark colored gun, holster, pants, and shirt, and the only one who saw it and busted him was a kid who, by physical design was staring right at his carry location.