This is well known to anyone who does scent work with their dog.
I do a game called k9 nosework with my corgi. You take a qtip dipped in certain scents and hide it in boxes in the beginning then progressing to furniture and outside areas. The game is for the dog to find the qtip and identify; there's even a contest / sport [1]. If you decide to do the sport, the handler has to read the dog and decide if the dog is identifying a given bit of car or whatever as the location of the goal scent or if the dog is just sniffing. The point of this rambling is even the plain trainers that do k9 nosework know that the handler has to go blind and can't know where the scent is or the dog will learn to read the handler instead of learning to use its nose to find the scent. The idea that the police are unaware of this is ludicrous, since this sport grew out of nose training for police dogs, and it's a major training obstacle.
When I was in high school I worked with k9 dogs a lot as an assistant trainer. This reminds me of a dog we had in that was deaf, but we had no idea until they hit the street.
We had trained the dog for 2-3 months just like any other k9 and everything seemed normal. The officer came for their week long training period and get to know the dog period everything was okay and any of the issues seemed minor and they were dismissed as just the dog and human learning to get to know each other.
Then the dog hit the streets and had tons of issues and we couldn't figure it out. Finally, after a bunch of trouble shooting we had the dogs hearing checked out. The dog was deaf, but how could it know the commands when it was in front of the officer or trainer? We're not 100% sure, but we noticed that based on the commands given and the person giving the commands often times they would assume a certain stance. Other times we found the trainer would bounce/not bounce the ball based the command given. The dog could feel the ball bouncing and tell the trainer had taken up a different stance based on the command.
I do a game called k9 nosework with my corgi. You take a qtip dipped in certain scents and hide it in boxes in the beginning then progressing to furniture and outside areas. The game is for the dog to find the qtip and identify; there's even a contest / sport [1]. If you decide to do the sport, the handler has to read the dog and decide if the dog is identifying a given bit of car or whatever as the location of the goal scent or if the dog is just sniffing. The point of this rambling is even the plain trainers that do k9 nosework know that the handler has to go blind and can't know where the scent is or the dog will learn to read the handler instead of learning to use its nose to find the scent. The idea that the police are unaware of this is ludicrous, since this sport grew out of nose training for police dogs, and it's a major training obstacle.
[1] http://www.funnosework.com/