I think you're missing that there is a sequence of things that happens here:

  1. he sets off the chemical detector
  2. he is pulled aside for questioning
  3. in that questioning, they discover he has a background that makes them nervous
  4. they hold him for a great deal more questioning

1->2 is standard and would happen to anyone. As other people have pointed out in this thread, it generally takes 15 minutes and is no big deal. It certainly doesn't involve the FBI. While it's never happened to me, it's happened to several people I know and while it was a bit of a hassle, it did not come anywhere near this.

The alarms have already gotten us to 2, and they're not even nervous yet, because they don't know his background and, like you said, without the background being true, the whole expression evaluates to false. That's why the 3->4 transition is a problem, and that's where the profiling comes in. The agent's explanation isn't some non-sequitur, like "yeah, this is totally standard and incidentally we're nervous". He's explaining why they're holding him longer and have brought in agents from five different departments instead of just having a TSA guy chat with him and send him on his way. Notice the escalation as they become more concerned; they care about his background because it makes them think he might be a terrorist. Holding people for questioning when they're nervous that the passenger is a terrorist is actually their job. That's why they did so much, even to the point that they felt the need to explain it.

If answers about his background would not change their behavior towards him, why would they ask about it all? The whole point of gathering information is so that you can make decisions with it.

So then we get to the crux of it: his background makes them nervous and their nervousness causes them to subject him to additional scrutiny, above and beyond the screening he would have endured had he set off the alarm and not had a nervousness-causing background. That's profiling.

I believe the issue with the alarm is that most people don't repeatedly flag the same alarm.

E.g. if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article, especially if the machine correctly doesn't alert on other innocuous articles.

So in the normal situation someone sets of the chemical detector, once, gets pulled aside for questioning. Their gear doesn't set off the detector again and so the agents are able to conclude it's a false alarm.

To be clear, I don't agree with the treatment OP received in this case, but I fail to see how it is sinister that someone repeatedly sets off chemical detector alarms (that no one else sets off repeatedly), has burned all ties to his residence, is religious and is going up to meet family for a religious gathering.

It's not that such behavior is automatically suspicious, it's that the behavior is still almost indistinguishable from those who previously have caused terrorist attacks.

In this case the FBI agent isn't trying to prove OP a terrorist as much as he's trying hard (and failing) to prove that he's not.

> if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article

I don't think that's true. Remember that the test doesn't actually detect bombs. It detects certain chemicals. Now, sometimes it might just randomly report a false positive and then you'd expect it not to trigger the second time. But some of the chemicals it is built to detect can be found in everyday products, such as hair products, soaps, some medications, and--as I learned today--bug spray. And so for those chemicals, it's going trigger repeatedly because the chemical it's looking for really is there. So that second failure mode is actually pretty common, and so even when that happens, it's normal to take the person into a side room, search them more throughly, talk to them for a few minutes, and send them on their way. What we see here is quite different.

Which leads us to the real reason, which you wrote:

> It's not that such behavior is automatically suspicious, it's that the behavior is still almost indistinguishable from those who previously have caused terrorist attacks.

That's precisely profiling: "the bad guys have profile x and you fit profile x, so we think you are suspicious" where x isn't inherently suspicious. It sounds like you're saying, "profiling isn't such a bad idea", which I strongly disagree with, but I suppose that's a different discussion.

> So that second failure mode is actually pretty common, and so even when that happens, it's normal to take the person into a side room, search them more throughly, talk to them for a few minutes, and send them on their way.

From what I am hearing from others, normally people barely even get that much special treatment after a non-transient false positive. Agents suggesting "Maybe it was 'Innocuous Product X'" so the passenger can say "Yup, that is probably it" seems to be common, but not the sort of treatment they are going to give to people that they have a bias against.

> E.g. if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article, especially if the machine correctly doesn't alert on other innocuous articles.

Well yes, I would. Some false positives can be transient, others are not. If I just came from the shooting range I would not be surprised if I got a non-transient false-positive. Now that is an obvious case and I would immediately tell them that I had been to the shooting range, likely resolving the issue, but it is just a simple example of a false positive that is not transient.

How common is a non-transient false-positive where the victim doesn't immediately have a good idea what the cause is? Well, there are several reports from HN users in this thread that describe situations in which it could allegedly occur. One cites hand lotion as a potential cause of non-transient false-positives; luckily for his wife the agents volunteered that hypothesis so she wasn't left guessing. The author of the article supposes that an over-the-counter chemical was the cause of his non-transient false positive; it's not like he was a lab tech working with synthesized stuff that nobody else ever comes into contact with.

Unless the machines are shit (a distinct possibility), I would expect transient false-positives to be relatively infrequent while non-transient false-positives would be reliably and regularly caused by a wide range of substances that share chemical properties with known explosives.

Most of these non-transient false positives are likely quickly resolved without much ado. His was not.

Edit:

Perhaps the real problem here is with the terminology. These machines are not really bomb detectors, or even explosive detectors. They are chemical detectors. Calling them bomb or explosive detectors is like calling a metal detector a "gun detector". Sure, finding those things may be why it is there, but that is not actually what it does.

> Well, there are several reports from HN users in this thread that describe situations in which it could allegedly occur

Which makes the detectors useless as an interrogation tool.

If the alarm sounds, then check the person for explosives. That makes perfect sense, it's a useful tool for finding explosives on people.

But if you cannot find explosives on the person, what do you do then?

Any even slightly training terrorist is going to know what other products would produce the same detection result as the bomb they just built. So they'll pretend to think for a while, and then say "I work in a supermarket, and a customer dropped hand lotion on the floor this morning, and I had to wipe it up... can that set off your machine?"

It seems like there's a magic answer you can give that will let you go free, you just need to know the right thing to say. If the investigator likes you (i.e. thinks you're probably not a terrorist) they'll give you hints about what you should say. If they don't then you're on your own.

Someone who can give the right answer is either:

* Good at analysis, so they can make a good guess of what might be setting of the machine.

* Someone who's been through this before

* Someone who got a friendly hint from the investigator

* A not-so-dumb terrorist

Someone who can't give the answer is either:

* A normal person

* A dumb terrorist

Given the low prevalence of terrorists, that would be the least likely explanation in either scenario, so the whole line of questioning is pretty much useless, except as a way of applying pressure to someone who you've decided is worth applying pressure to. When that decision is based on some genuine piece of evidence, then it might be a legitimate law enforcement technique. When it's based on the gut-feel of the officers in question, it becomes a front for racial profiling.

> if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article

The last time I was travelling, I set of the metal detector in an airport in Germany. They took me aside and used the hand metal detector. It also went off. So they patted me down and scanned me again - the alarm still went off. So they patted me down a second time and scanned me again and the alarm still went off.

They then sent me on my way and I had no further hassle for the rest of the trip.

I understand what you mean now. Your original statement was:

  They even admitted to subjecting him to additional security because
  of his "background".

This is ambiguous and can be interpreted in more than one way. How I interpreted what you said:

  1. He sets off the chemical detector
  2. He is pulled aside for "additional security"

You can see why I disagreed as anybody ought to be pulled aside for additional security if they set off an explosives detector. Upon reading your reply, the interpretation you were going for was:

  1. He sets off the chemical detector
  2. He is pulled aside for "additional security"
  3. They probe into his background to build a profile
  4. This profile yields further rounds of "additional security"
     that people not matching that profile wouldn't be subject to

I lack knowledge on whether or not making security-related decisions based on a profile is useful or irrelevant, so I'll bow out of this aspect of the conversation.

Ah, I see. Yeah, I meant, "additional" as in "above and beyond the standard reaction to setting off the detector". Sorry for the confusion.