You mention Will knowing Andrew having a gun. People do have tells. There are tells of the unskilled and tells even skilled people make. An unskilled tell is patting where a gun is carried. A skilled tell is keeping the dominant hand free and in certain positions. Some move in distinctive ways due to intensive training.
Some years ago, before the fall of the Soviet Union, I was at an exhibition of The Treasures of the Hermitage with a friend. For fun I was pointing out the KGB guys in the crowd.
"How can you tell?"
"They all move the same way. Once you learn to spot it, it's like a neon sign. They all learn it at the academy."