It's tricky because, while dogs can understand fairly complex vocabulary, their ability to reason abstractly about events is less developed. If the dog alerts,

It's tricky because, while dogs can understand fairly complex vocabulary, their ability to reason abstractly about events is less developed. If the dog alerts, you spend two minutes searching, and then you reward the dog, it will have trouble connecting the reward to the alert. The obvious solution is to train the dog to search things that you know have drugs (or don't) because the trainer set it up that way. You'll need to then repeat this training often, and not always in the same environment. If you don't, eventually the dog will figure out that it will always be rewarded for alerting in the field. The problem with this, of course, is that it's expensive and cumbersome to do. You need to set up cars/bags/lockers/etc with and without drugs in them in a variety of locations, secure them (lest actual criminals steal the drugs), and bring the dogs to them (lots of time out of the handler's day). Also, even if you do this, you'll still get false positives: dogs are living animals, not machines.