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I've noticed that occasionally when passing through the metal scanners I am getting flagged despite not having a single metal thing on me. This usually results in a manual search or a hand swipe on one of the explosives detectors.

Is this being done on purpose to give an excuse for extra checks? Or perhaps the button on my jeans or some other tiny piece of metal is triggering the alarm?

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    Followup: Assuming the random trigger is true, what is the point of doing that? – Mindwin May 20 '17 at 7:55
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    @Mindwin: Let's say you manage to get your hands on a mostly-plastic gun, or fill your shoes with explosives, ... well, there's still maybe 1 chance out of 5 that'll you get hand-checked at the gate. That's enough to make any terrorist/hi-jacker wannabe (1) think twice and (2) get really nervous at the time of actually trying it. In the same vein, I once saw a "gun" appearing in a friend's carry-on. It disappeared when the agent pressed a button, so I guess it's a way to make sure the agents stay on their toes too ;) – Matthieu M. May 20 '17 at 18:43
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    @Mindwin Or it's to give the illusion of randomness to a search the security wish to perform for any other reason (race, behavior, random search quota). So much easier to put people on the spot and blame a machine than deal with accusations of prejudice. it's one of those things that make so much sense that if it isn't being done already, it will be. Or actual randomness to keep security on their toes. – kaay May 23 '17 at 13:32
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Sometimes.

At least in the TSA Precheck lanes in the United States, the metal detectors (Precheck lanes use metal detectors, not body scanners) can alarm randomly to alert the screeners to perform a random check (you can read various anecdotal experiences in this forum thread). Some people report it happening frequently enough to them that they suspect it's less than random, though I know of no definitive proof of this, and the nature of random events is such that some people will feel singled out regardless.

In my experience, the TSA screener at the lane can tell the difference between an alarm caused by the presence of metal (they'll tell you to check your pockets or take off your watch or belt or whatever) and one caused by the randomizer. In some cases, they'll stop you and tell you that it's random as you start to check your pockets. If the random alarm goes off, they usually do a hand swab or a brief pat-down or some other additional check; I've heard of people getting sent to the body scanner as well.


While the exact configuration of airport security equipment is typically confidential information, it's not uncommon for the manufactures of walk-through metal detectors to offer an optional "programmable Random Alarm capability." In the case of one particular model, "Random alarms are indicated by a pulsed alarm audio and zone lights sequencing from top to bottom." The likelihood of a random alarm can be adjusted on the control panel.

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    "less than random": a high frequency doesn't imply a lack of randomness. – phoog May 17 '17 at 17:27
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    Well yes. If you have a good random generator which generate integer numbers between 1-100 with an appropriately high Kolmogorov complexity and then you feed this sequence into a routine which triggers the gate for all numbers less than 96 then a) aye, it's random b) very high frequency. – chx May 17 '17 at 18:31
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    @phoog - if a traveler triggers the "random" alert at a higher frequency than other travelers, then that does imply that the alert is not random. It doesn't prove it, but it does imply it. If after 10 flights I randomly triggered the thing 7 times, while my traveling companion triggered it only once, then someone may very well be triggering it based on my appearance or other factor. – Johnny May 17 '17 at 22:44
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    @Johnny The one issue with that analysis if that if it is a random one in ten event then the unlucky one in ten million person who gets pulled up 7 times in a row will find that most of her travelling companions have only been pulled up once or less. She'd be justified in thinking it's not random, but might still be wrong. – origimbo May 18 '17 at 0:29
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    Doing the stats on the probabiltiy of at least one passenger in all of air-traffic getting stopped N times in a row is fascinating. 895 million passengers does an amazing job of making apparently statistically unlikely events surprisingly likely. – Cort Ammon - Reinstate Monica May 18 '17 at 0:36
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This is a definite yes. If the metal detector beeps, turn around and you'll see "QUOT" for quota if you've been selected for random screening.

Edit: I paid particular attention to this on a trip this weekend. The QUOT method was happening at LHR but in IST they had a triple beep and a strobing light. When selected for random check they didn't wand the person at all, but just took hand swabs. A small child was randomly selected and they just let him through without doing anything.

When the alarm went off for real, it was a single beep and a light indicated the location of the alarm. If it was at foot level, they'd tell you to take your shoes off and put them through the x-ray, otherwise they'd wand they'd perform a hand search.

So if you're particularly observant you can probably tell by the beep, but if you don't catch that, you'll have a second or two to turn around and see the alarm indicator - real/random.

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    You get my vote for pointing out "QUOT". I was passing through Heathrow a week ago and a solid 10-15 people in a row got beeped and sent to the millimeter wave scanner. I thought something was wrong with the metal detector, but I didn't see it pointing out metal with its lights, and saw QUOT flash every time. Must have been some random "do a chunk in a row" program. – mbrig May 19 '17 at 2:52
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    @mbrig Heathrow is the worst for this. They turn up the settings to the nth degree. I first noticed it when my young son who definitely had no metal on him, set off the metal detector. Now I turn around whenever the alarm goes off to see why. – Berwyn May 19 '17 at 3:03
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    I didn't even know until maybe 6 months ago that the lights flash at the level where metal is detected. At least now I can tell which pocket I (probably) forgot something in :) – mbrig May 19 '17 at 3:10
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    I don't see the point in telling the staff that it's a QUOT alarm; isn't the risk that a bad guy will not be checked out manually as thoroughly? – bye May 19 '17 at 12:13
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    @DrEval knowing that it's a random means that if their secondary screening doesn't turn up something they don't have to freak out that they've missed something, that the passenger could indeed be clean. Otherwise they'd have to waste more time trying to figure out why one machine triggers but the other doesn't. – Doktor J May 21 '17 at 22:25
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I cannot speak for other countries, but in Canada, the metal detectors used at the airports also beep randomly to indicate a further search. Usually, they let the passenger choose between a pat-down, scanner, or swab.

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    They let the passenger choose? How does that exactly work: "Step aside please. Would you rather have me fiddle your parts or walk through that scanner" ? – Dusan Bajic May 18 '17 at 8:28
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    Yes, exactly. That, or "swab your hands and bag". I always choose the swab. – Martin Argerami May 18 '17 at 12:19
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    Interesting :), I have never been offered to choose (but have also never flown to North America) . – Dusan Bajic May 18 '17 at 12:27
  • May you include a citation please? – EKons May 18 '17 at 17:32
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    I assumed that the machine beeps - detecting the gun - and then the officer offers the three choices, as described. Handling a gun and ammunition does not get you in direct contact with gun powder, unless you make your rounds yourself. Regarding the latter, see latimes.com/local/lanow/… – Aganju May 19 '17 at 11:27
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Since the likelyhood of a scanned person carrying a forbidden object (like a knife or a weapon) is low, the most likely result of sitting hours watching a screen or a queue is boredom and loss of attention.

Therefore having the software randomly raising an alarm helps in keeping the personnel effective.

I remember reading somewhere that it also happen with hand baggage scanner.

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    Where did you read this? It would be interesting to find a link. – JoErNanO May 18 '17 at 13:50
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    The man himself: "Pat-down procedures are used to determine whether prohibited items or other threats to transportation security are concealed on the person. You may be required to undergo a pat-down procedure if the screening technology alarms, as part of random or unpredictable security measures, for enhanced screening, or as an alternative to other types of screening, such as advanced imaging technology screening. Even passengers who normally receive expedited screening, such as TSA Pre✓® passengers, may at times receive a pat-down." – David Tonhofer May 18 '17 at 20:45
  • This answer strongly contradicts my experience. Every time I go through security, I see them finding possibly suspicious items (we are talking a screwdriver or a pocket knife, or a bottle of water) every two or three passengers. – Martin Argerami May 19 '17 at 20:56
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Anecdotally (passing through a friendly airport at a quiet time -- UK) and setting a metal-detector arch off despite not having a scrap of metal on me¹ I was told that they sometimes just go off. The implication from staff was that they're set up to be so sensitive they sometimes alert for no real reason.

¹Shoes removed, even watch and wedding ring in my bag, and no metal in my body; I tend to fly in clothing with lots of pockets and no metal fasteners.

1

Technically, NO. They do not sound a false positive. What they do is sound an alternative alert to indicate the person has been 'selected' for additional screening.

For clarity, actual false positives can occur because the machine detected something or is not calibrated correctly.

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