They just key in on people the handler-cop is suspicious of.
pandagon.net - it's the eye of the panda, it's the thrill of the bite
Sebastian recounts research showing that dogs' tendency to signal has more to do with what the cop is thinking than what the dog is smelling. Anyone who knows dogs should have guessed this one; dogs are basically human-obsessed machines who watch their humans super carefully and try very hard to please them. Of course drug dogs are more worried about pleasing master than producing good results. The real world results are predictable, but no less upsetting for it:
A tracking study was done of drug sniffing dogs in Illinois which found that the searches their 'alerts' triggered found no evidence of drugs 56% of the time. For Hispanic people searched as a result of the 'alerts' there was no evidence of drugs 63% of the time.
You can read about it at the Chicago Tribune. The cops are pulling the "well, they're guilty of something" bullshit, saying the dogs are smelling drugs that used to be there. Maybe. But again, I point to the well-documented Clever Hans effect and suggest that it's something else entirely, which is that the dogs are picking up on the officers' prejudices and acting accordingly.
Obviously, the ultimate goal here is to call off the War on (Some People Who Use) Drugs, which is run on magic and bigotry, and does more to destroy communities than to prevent drug addiction. But in the more immediate future, we must demand an immediate end to all use of drug dogs, certainly until it can be demonstrated in double blind studies run by experts that the dogs are detecting drugs and not reacting to subconscious signals sent by police. Since I highly doubt that can be demonstrated, basically I'm saying that drug dogs should be permanently banned. Even if they worked, they're basically a cheap attempt by law enforcement to skirt constitutional protections, but since they don't even work, they're nothing but a magic trick used to distract from what's really going on: cops conducting illegal searches based on their own prejudices.