A friend of mine was recently pulled over at Gatwick on the grounds that traces of explosive were found on his shoes. Turns out he'd been at a clay shoot the day before and walked through the area in front of the gun stands, where unburnt propellant fell.
This oversensitivity of detectors leaves gaping holes in (so-called) "security" measures. For example, as an activist for the Half-past four in the afternoon faction of the People's Liberation Front" you wish to get publicity. A good idea for plenty of press coverage is to cause a "security" "alert" at a major airport. Get a 12-bore cartridge and remove the propellant (we now know that the detectors are sensitive to cordite). Grind up fine in a mortar and place in small plastic bags. Wearing minimal disguise, enter the check-in area and unobtrusively scatter the powder on the baggage of people queueing to check in. Remember you are under CCTV snooping, so go to an airline desk and make some routine enquiry. Then leave.
When your first victim finally reaches a check-in desk, their luggage goes on the belt and triggers the detectors in the baggage handling area. Pull baggage and find the owners. Restart the belts. Next baggage triggers the detectors. Same result. After three or four times (some, no doubt, caused by contamination of the belt) the Authorities have the unenviable choice of either deciding they are under a major attack and must therefore close the airport, or assuming the detectors are faulty, keeping quiet and letting everything through.
Similarly New "Scientist" recently carried a breathless piece about a new ultrasensitive explosives detector that could pick up traces on car door handles to enable detection of car bombs.
Let's think about this. In those parts of the world where car bombs are a genuine threat, hawkers and children frequently ply their trades in traffic jams, such as those queueing for a check-point. How simple to go up and down the queue with explosives on your hands, touching everybody's door handles?
Careful Disputin, talk like that will get PRISM focusing on you. Especially you, since your information is "external" to the US.
I was pulled over for a random search of my car at Dover/Folkstone (I have been so treated twice that I recall. I must fit someone's profile).
They did the open the boot routine and wipe all the surfaces with a gloved hand then put the glove in the sniffer machine.
I was very fortunate not to be making this trip too closely after a call I had made to the then operational (is it still?) factory in Somerset where they make plastic explosives. I was right in the heart peering at their vats and generally given a walk through of the process.
Now that would have given some one some fun that day..... but what about all those who actually work in such areas? Do they have to carry a card like Thyroid treatment patients who have to announce they are radioactive - before SWAT descends?
I suspect no card since 99.999% of tests are negative and the overwhelming majority of positives are false positives, such as those who have been around shoots or are legally carrying ammunition.
Has airport security sways ever actually worked, in terms of catching persons carrying explosives? The answer will of course be a wrong number, since the purpose is deterrence rather than detection.
How do you deter someone from blowing himself up?
The purpose of all these outrages boils down to publicity, so the logical way to fight it is to get the meejah to report "An aircraft crashed today. It was believed to be a bomb", but no way will they do that. Remember when Margaret Thatcher forbade the broadcasting of IRA statements? The BBC smirkingly had an actor (they said) read them out.