Explaining scent detection dogs in short

Scent detection dogs like the search and rescue dogs from 9.11, explosive and narcotics K9s, diabetic alert dogs, and so on all share one thing: they live to find and detect. All these dogs get trained to find a specific target scent, aka odor, and once they locate and properly indicate that they found their particular odor, they get rewarded. Usually, that reward is a play session with their favorite toy. Toys are used, because out in the field, on a mission, it is easier for the handler to carry a reward toy than reward food. Sniffing pays; these dogs learn that when they put their nose on the target odor, it will pay off majorly. 
Sometimes, these may look sad or “too happy” to the bystander, but that reflects human emotions in the actual situation. These dogs may look sad because they haven’t been able to locate what they have been trained to find, which then, of course, means these dogs won’t get their favorite toy to play with. Sometimes, these dogs look overly excited when they make a find (strange in the case of a human remains dog), but for the dog, that means he or she did great, and they get to play and get rewarded. Dogs, just like humans, get tired, and having to engage and use their noses for long periods of time is very mentally exhausting for dogs due to the sniffing and strategic thinking that is happening at the same time. If this kind of effort stays unrewarded for long periods of time, it can be very emotionally taxing for the dog hence the dog may look sad or even depressed. Just picture yourself working all day long, and you don’t see any significant outcome; that would make you sad too. 
It's important to remember that these dogs, while they may have saved lives or prevented crimes, do not understand the significance of their actions. They are not aware that they are heroes. All they know is that their hard work, in the form of detecting a specific odor, is rewarded with playtime with their favorite toy and their favorite human. 
Just that, they are heroes without knowing that they are heroes. Just there for the moment.