What did I do to set off a siren at the automated e-passport gates in Heathrow arrivals? [closed]

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.