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Earlier today as I put my passport in an automated e-passport gate on the way back into the UK at Heathrow Terminal 3, a siren went off (!). What could that mean?

  • Did I make a mistake using the machine - standing in the wrong place, or pushing my passport against the scanner too hard? (The lady overseeing the machines had told me to take my glasses off before I went in, so I didn't really know what I was doing.)
  • Or did the machine think I didn't match my passport? I'd have thought they'd do that more discreetly.
  • Or could it mean I've been flagged somehow and they want to see me in person? Ditto I'd expect more discretion.

Embarrassingly on the way out of the UK last week I had forgotten about a 500ml (sealed, untouched) bottle of diet coke in my hand luggage that got caught by the bag scanner. Would they have recorded my passport number then, and could this be the fall-out from that?

I was sent around the corner to see an actual person, who looked at my passport, asked where I'd flown from, then let me through without any more hassle.

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  • The system, which was then on trial at Manchester Airport, would allow two people inside the booths without triggering an alarm. telegraph.co.uk/news/uknews/crime/7908260/… I believe your question though interesting could have many possible reasons and lend itself to conjecture.
    – Augustine of Hippo
    Commented Mar 27, 2017 at 14:25
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    There's probably all kinds of reasons for the system to divert a traveler to a manual check, ranging from a hold on the passport number, to the facial-recognition software being confused and not reaching a positive match (to the degree of certainty configured for it), to not being sure there is only one person in the booth, to deliberate randomness (to keep border guards on their toes and make sure being diverted is not in itself a black mark). It's probable that the floor staff doesn't even know what's behind any given alarm, unless it's a passport flag.
    – hmakholm left over Monica
    Commented Mar 27, 2017 at 14:35
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    The electronic chip in the passport is encrypted. It cannot be read without the key, which comprises data from the machine readable zone. So the passport has to be opened and then either scanned optically or read by a human before the RFID chip can be read. Facial recognition is perhaps more plausible, but the number of people forgetting that they have drinks in their bags must be truly huge, and flagging people for that would be an ineffective use of resources.
    – phoog
    Commented Mar 27, 2017 at 15:37
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    This seems answerable to me. Even though we can't know why this particular traveller received a siren, it seems plausible that somebody would know a list of reasons that cause a siren to sound. I don't recall any siren when I was sent for manual contro (the facial recognition couldn't cope with my glasses and I'm short-sighted enough that I couldn't see what it was telling me to do, without them).
    – David Richerby
    Commented Apr 5, 2017 at 7:53
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    @DavidRicherby agreed. Let's keep this open.
    – JonathanReez ♦
    Commented Apr 5, 2017 at 10:55

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