Do airport metal scanners occasionally flash a false alarm on purpose?

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.

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.

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.

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.

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.