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Before entering a commercial plane, I had to go through the usual security check at Prague airport (PRG). When I passed the metal detector, it went off. I assume it was because I forgot to take off my wedding ring.

I was approached by the security guy and instead of a full check (which I was expecting), the security guy marked both sides of my hands with some piece of wet paper, told me "wait there", left with that paper (maybe checking something with the paper, I do not know exactly) and after some time he told me I was free to proceed to the Duty Free zone behind the security check.

While relieved, I am wondering: Does anyone have a clue what kind of security check was that?

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    That is explosive and/or drug testing. I had the same happen to me in PRG.
    – JoErNanO ♦
    Commented Feb 9, 2016 at 13:39
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    The likelihood of the metal detector being set off by a wedding ring is extremely low. I travel for work (flying every two weeks) and I never take my ring off for security. There is a random sample of passengers who are selected for extra screening, irrespective of metal detection. I don't know about Prague airport, but the machines at Heathrow and Stansted in the UK have a different bleep for that. If there is a single bleep that sustains as long as you are in the machine, it has detected metal. If it bleeps a few times then it has just selected person <N> for screening.
    – ssmart
    Commented Feb 9, 2016 at 15:20
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    The basic quick answer is swabbing for bomb making residue.
    – D Bell
    Commented Feb 9, 2016 at 23:54
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    Given all the comments about false positives, rather than calling this an exposives test, they should call it; A test for chemicals, some of which are found in explosives but often are found in innocuous everyday materials.
    – Oscar Bravo
    Commented Feb 10, 2016 at 9:06
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    I had a job where I handled all manner of explosives quite frequently. My backpack, keychain, tools, laptop and even some clothes I had never worn to work would frequently swab hot at airports. Not all of these systems are very good, but quite a few of them are really sensitive and actually do work.
    – zxq9
    Commented Feb 11, 2016 at 11:53

4 Answers 4

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Often at security they swab my backpack and some of my possessions with a small piece of damp paper. It is supposed to pick up traces of the materials I've been handling. Then they put it in a machine that analyzes those traces. At customs and immigration it's set to detect various illicit drugs (I've seen this on Border Security) but at security I believe it's set to detect explosives.

Certain people are chosen randomly for a swab. Since you had no residue on your hands that would require them to inspect you further, they sent you on your way. This wasn't caused by you leaving a ring on. Leaving a wedding ring on is fine.

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    @Insane, actually, washing your hands is not all that efficient. It will always leave some percent of the traces and if the test is sufficiently sensitive (which it is), it can probably pick the traces after washing your hands a couple of times. The kind of sensitivity was available in chemical labs for years; they just had to put it into sufficiently automated device so they can operate it without a chemistry major.
    – Jan Hudec
    Commented Feb 9, 2016 at 16:51
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    @Insane further, they typically swab stuff you've been touching with the theory that you touched that stuff before you washed your hands. Not foolproof, but since it's quick, they can do it a lot, and perhaps it occasionally turns something up.
    – Kate Gregory
    Commented Feb 9, 2016 at 17:00
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    @Insane, we are saying that the measure is effective and I believe that is generally OK, because part of the effectiveness is that it forces the potential wrongdoers to complicate their plans, which increases the risk of failure or disclosure at other points (the shoe and slips bombing attempts are considered example of this; they managed to make bombs small enough to evade detection, but that just made them too small to actually cause significant damage).
    – Jan Hudec
    Commented Feb 9, 2016 at 17:58
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    @Insane Besides what everyone else mentioned about washing, it is also worth noting that doing this might actually make it more likely for you to test positive. If you use soap or lotion that has glycerine, you can actually test positive for explosives and get a (very thorough...) searching. This just happened to a good friend of mine last month when we were traveling out of country. She had just applied lotion before going through security and earned herself a full cavity search.
    – LoganGoesPlaces
    Commented Feb 9, 2016 at 18:05
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    Sometimes it doesn't even have to be something you have applied. I used to work at a business lounge where were served miners. The soap we used was glycerine free and was used because it didn't set the detectors off. One day after an 11hr shift, I tested positive for TNT coming back through security (forgot my keys). They re-did the test and it disappeared. Sometimes trace can be left on you from others, the air or things you have brushed passed. Likewise so can the detector pad.
    – The Wandering Coder
    Commented Feb 10, 2016 at 4:20
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The swabs are used to check for explosives and/or drugs. Indeed, if you manipulated any such substances in the near past, it's likely that traces can still be found on your hands, and on the things your hands usually touch: bag handles, zippers, pockets, etc. If you pay close attention during the checks, these parts of your personal belongings are the ones that are usually swabbed. The TSA blog calls these swabs state of the art Explosives Trace Detection (ETD) tests:

While going through checkpoints, you might have seen officers using little white swabs at TSA checkpoints at one point or another. In case you had no idea what our officers were doing, they were conducting state of the art Explosives Trace Detection (ETD) tests. And all along you thought they were giving your items a complimentary cleaning…

ETD tests are used in checkpoint, checked baggage, and cargo environments. We swab things such as laptops, shoes, film, cell phones, bags, wheelchairs, hands, casts - you name it. Certain procedures call for an ETD test.

Basically, our officers run the white swab over the area in question to collect a trace sample. They then place the swab in the ETD machinery which analyzes the sample for extremely small traces of explosives. The test takes a matter of seconds.

[...]

So as you travel, you might be asked for a swab of your hands at the checkpoint or gate. It’s painless and quick. The swabs are disposed of after each use and will not be used on more than one person.

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This is a way to check if you have manipulated explosives recently. The paper will absorb the particles and then react during their test afterwards.

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Something that hasn't been noted by others is that while this is testing for explosives, it can also pick up residue from firearms usage e.g. if you've been shooting and/or handling ammunition, particularly spent casings like shotgun shells it can also cause these tests to come out positive.

If you have been shooting say at a range or you've been clay-pigeon shooting etc. it could be a good idea to take a receipt for that to the airport with you to avoid misunderstandings.

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    what would prevent the guy with a real bomb to also go to a shooting range before and get a receipt? Maybe showing a receipt will make the security more suspicious, because why is this guy so well prepared for this that he even saved the receipt from the shooting range? Most people throw away or lose their receipts quite soon.
    – lowtoxin
    Commented Jan 7, 2017 at 8:39

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