Aye Dave's bro, but consider this, purely hypothetical scenario, thrown out merely to provoke comment on why it is wrong:
Airport security screeners have for years been performing a useful service: making sure nutjobs don't blow up or hijack airplanes. All of a sudden, thanks to a series of geopolitical events brought about by what has unfairly been construed as a failure on the part of airport security (blades that length were allowable on board), security finds its mandate, and its ranks, expanded. So, in one blow, you increase the security responsibilities and you decrease the entry requirements. Worse, a previously "neutral" civil-service-style job, has become a politically "charged" one, where the employees are rewarded for their achievements. Ideally, it's someone with plastique in a toothpaste tube, but since that hasn't happened, the next best thing will work. For the paid informants, that's a matter of trapping someone into saying something that might be compromising (but inevitably gets thrown out of court); for the security screeners, it's grandmas with knitting needles and pilots who might smell of aldehydes. Oh yeah, I'm sure a case in the US with genuine hero security screeners spotting a couple of pickled pilots didn't help.
There's the problem: security screeners who are told that "since 9/11, everything depends on you", and encouraged to do things beyond their training. So we get the UK, which now has two highly-publicized incidents of "Pilot Intoxication", one of which results in a not guilty verdict; the other doesn't even make charges. Sorry guys, it's clear: Either the security examiners are explicitly trained to judge pilot fitness, and this job is publicly made known, or they're made known that, if they are going to accuse, they're doing so in the capacity of any other private individual at the airport, only they're putting their job at risk.
Hey, choices of conscience are hard, in part because the penalties are severe. But -- and this is where I bring my hypothetical BS in -- if the system gives individuals benefit from detecting the immeasurably rare, then the system will artificially generate what it claims to measure. And that increases misery and decreases judicial efficiency.