I was referring to dogs and explosive sniffing machines (not WBI scanners) as the first line of detection for explosives, if such means were consistently used. What kind of false positive and false negative rates would such methods have?

Obviously, in any positive, some sort of secondary screening would be indicated to distinguish between a false positive and a true positive, but that gets ugly, as an intrusive pat down (or WBI scan) would be considered offensive by many people.