I've always wondered how police dogs are trained to track people down and sniff out certain drugs. I have a friend whose dad used to be a police officer, and I was told that it wasn't an easy process to train those types of dogs. I can definitely imagine. However, after months of training, I'm sure the dogs becomes used to whatever they've become assigned to. Just like with any other forms of training, a dog will know when to sit or stop barking. Sometimes, you don't even have to tell them to do so.