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.
travel.stackexchange.com/users/13650/Zach Lipton
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.
travel.stackexchange.com/users/43570/Berwyn
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.
travel.stackexchange.com/users/10087/Martin Argerami
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.
travel.stackexchange.com/users/61311/A traveller
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.
travel.stackexchange.com/users/18491/Chris H
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.
travel.stackexchange.com/users/46803/Johns-305
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?
Do airport metal scanners occasionally flash a false alarm on purpose?
travel.stackexchange.com
<airport-security><security>
travel.stackexchange.com/questions/93463
travel.stackexchange.com/users/9009/JonathanReez
There could be a false positive due to incorrect calibration or a notification to perform a random check, which can be adjusted to occur more or less frequently. If it is a random check, this may be indicated on the machine or you may be told about it. In some cases the scanners are designed to randomly select individuals  to search and in others the scanners are designed to be so sensitive it picks up most  things, most of which may be irrelevant.
In some cases the scanners are designed to initiate random checks based off of a coded program or preset quota In some cases the machines are extremely sensitive and do not sound off randomly, but are instead set off because something is detected 