How do police train dogs to find hidden drugs?

The test involved a whole bunch of volunteers to line up at the empty (because no international flights were due in for another half hour) immigration counters as if they were normal every-day passengers. The police had a cap of oil (capsule of marijuana oil) wrapped up in tinfoil, which was in turn wrapped up in two large plastic bags with rubber bands holding it all together. They put this down the sock of one of the volunteers, and everyone was asked to just ignore the dog as it came around.

The dog was led up and down one isle and then the next, but was extremely keen to head for the fourth where the drugs were. The handler kept the dog on track, sniffing everyone and their "luggage" (any hand-held items as we weren't real passengers) until they reached the fourth row. Then the dog bolted for the guy with the drugs, nearly tearing the arm off the handler. The dog jumped up once on the volunteer with the drugs, but was pulled back, and then sat beside him staring at him.

The handler pointed at the volunteer, looking at the testers, and said "This guy's got drugs". The testers went over and got the drugs out of the volunteer's sock, and said something about it being a low-odour sample too, and we all went our separate ways. All up, an extremely impressive display of the power of a dog's nose.

On another note, the bomb-detector dog was often sniffing around our offices as a bit of practice too. They had wrapped samples of gunpowder and other explosives that they hid in various locations (where no humans would accidentally find them) and get the dog to sniff them out.

The most common way to train dogs is to make it a game.

The dog is given a toy. Some little thing that will amuse the dog for the rest of its life if the dog doesn't swallow it. Then the toy is taken away, and painted with the scent of one of the dog's responsibilities, such as a certain drug or explosive. Then the toy is hidden in a mock environment and it learns to seek out the scent of this drug or explosive. After a little bit of this, real drugs or explosives are hidden instead of the toy. The dog may be surprised to find this, but then is reassured of its success when it is given the toy after finding the real thing.

Much time is needed to train the dogs to be suspicious, and go after possibly masked scents, such as drugs packed in coffee. This is done by leading the dog to the area of a masked scent and guiding them to it; then redoing the exercise without giving the dog any guidance. The dog must also learn only to search on cue, since the dog needs to remain calm around the Police Evidence Room or the Crime Scene Investigator's truck.

Dogs must also eventually be taught not to pick up or touch the target, as in the case of a bomb this could be dangerous and in other cases it wouldn't be proper for a police dog to accidentally indulge in some of nature's finest.