Dogs smell things differently than humans. If you are cooking spaghetti sauce and a human comes in to the kitchen they identify the "smell of Italian" food. A dog, upon coming into the same kitchen, identifies tomatoes, garlic, onions, parsley, oregano, etc. as individuals. If a dog is trained to hunt down parsley, no matter how much garlic may be used in this spaghetti sauce the dog will always be able to identify even a small amount of parsley. -- This is why dogs can find drugs even if they are stuffed inside of a gasoline tank, or packed with coffee or peanut butter. I've have seen a former state trooper k-9 unit officer explain this.
For a dog to be trained to that one smell the trainer puts drugs, or explosives, or whatever inside of a toy ball which the dog plays with regularly; to get the dog to find drugs/explosives the trainer merely tells the dog to find its toy. If you have ever seen or had a dog that ALWAYS wants you to throw him a ball or stick etc no matter how much you play with it, you have seen a dog which can easily be trained to search things out. Other dogs take a lot more work to train.
For a time I was very interested in working with dogs like this.