In other words, the test is almost completely useless. Given how low the base rate of terrorists sneaking through bombs is, a positive test result on one of these machines is >99.9% likely to be a false positive.

Let's do the math. We'll assume there are never any false negatives and just look at the positive results. Let's simplify "0.6-1.8%" and just call that "1%". Out of 100,000 bags, let's assume that ONE contains a bomb being snuck through by a terrorist. 1/100,000 is our postulated base rate of terrorism. 1/100 is our false positive rate on the test.

So let's put 100,000 bags through the machine. There will be 1,000 false positives and one true positive. which means that if some bag "tests positive for explosives" the odds are a-thousand-to-one against that being a valid result.

"But," I hear you cry, "we RUN IT THROUGH AGAIN when we get a positive result!"

Sure, that would work GREAT if false positive results were COMPLETELY RANDOM. But they're not. More likely than not, the false positive is being triggered by something that is or was actually in the bag. So when you run it again, there's a good chance that it'll trigger again. The "false" part of "false positive" is that the thing in the bag that it's triggering on...isn't an explosive. It's just some other chemical.