Honestly, dogs shouldn't even be used except in certain situations, for the post part, their findings should be as inadmissable as a polygraph because; and I want to be clear IN THE WAY THEY ARE COMMONLY USED they are little more than a prop.
The reason for this is while, they have excellent snouts, they are even better at playing clever hans.
So if you have an endless line of luggage to check, or lines of random cars waiting.... that is, situations where the handler himself has no reason to suspect anything in any particular place, dogs perform quite well, they are excellent sniffers.
However, its been shown that in cases where there is any suspicion at all on the part of their handler, that a dogs false positive rate goes through the roof to the point that they actually "hit" on nothing more than their handlers pre-existing suspicion more often than not.
Essentially maning, dogs are worst than useless in the most common use cases, and really work best in the rather uncommon cases of tracking and large scale checkpoints; and have little to no place at all anywhere else.