I'm no expert on this, but my understanding is that scent dogs do not work continuously. Instead, they sniff out for short periods when ordered to do so. However, because these dogs tend to be highly motivated, I could see the possibility of one of them alerting on a smell in order to get a reward. But on the other hand, these dogs, being highly trained, have been strongly conditioned to expect rewards *only* when they are following commands. When training a dog, particularly at that level, it is very important that the dog perform a behavior only when it's commanded to do so. Otherwise, you get what's called "default behaviors", where the dog performs it's "favorite" behavior over and over in the hope of getting a reward.