Contested in some quarters, air passenger profiling is gaining ground among watchdogs as a defence against terror attacks following two bloody assaults in less than a month.
The technique entails closely scrutinising passengers for identity or behavioural clues, backing classic methods such as baggage and body scans.
At Tel Aviv's Ben Gurion International Airport, widely considered to be the world's most secure, all passengers waiting to check in are questioned by security agents who ask questions about their trip, looking for nerves or inconsistent statements.
If replies raise suspicions, the passenger is singled out for additional screening which can involve hours of questioning and an exhaustive search of their luggage.
European security experts, at a two-day conference in Barcelona this week, said they were reluctant to recommend going that far, due to the amount of time it would take and the high cost for airports.
On the other hand, behavioural profiling is a useful tool that is widening in use, they said.
Profiling behaviours consists of “detecting abnormal behaviours throughout a passenger's entire trajectory”, from the moment they enter the airport until they board a plane, said Erick Bourai, who is charged with security at Airports Council International Europe.
“Observation is done first at a distance, by specialists,” who then question a passenger if they notice anything out of the ordinary, he told AFP on the sidelines of the two-day conference which wrapped up in Barcelona on Wednesday.
A coordinated wave of attacks on Paris restaurants, a concert hall and France's national stadium on November 13th left 129 people dead and more than 350 injured.
On October 31st, a Russian airliner was downed over Egypt's Sinai peninsula, killing all 224 onboard in what Russia said was a bomb attack.
“Technologies come and go, but our capacity to detect anomalies in human behaviour will never change,” said Lauren Stover, head of security at Miami International Airport.
A passenger who appears to avoid security guards, who sweats a lot or is wearing a heavy coat while waiting to board a flight to the Caribbean would all draw attention, she said.
“A single behaviour is not enough, it is the sum of little details,” said Ruben Jimenez, head of security at Geneva airport which will start training security guards on how to analyse behaviour in 2017.
All airport staff, from “duty free” cashiers to waiters at restaurants, can help detect suspicious attitudes, said Stover.
At Miami International Airport, which employs 38,000 people, police have since 2005 received a week of training in how to spot abnormal behaviour, she said.
Security guards receive a day of training while other staff attend a one-hour session on profiling techniques.
Civil liberties campaigners have warned of the risk of bias or prejudice, if profilers single out people on the basis of religion, ethnicity or skin colour.
The risks were underscored on Monday when officials at Paris' Orly airport insisted on searching an Algerian minister even though he holds a diplomatic passport, leading to an official protest from Algiers.
Jimenez said that on practical grounds alone, selectively choosing certain stereotypes for profiling would be a flaw in a security screen. The best surveillance had an unpredictable factor, he said.
“If you take a plane twice, you know, you know how security controls work,” he said.
“When I board a plane, I shouldn't know if they are going to question me, make me go through an explosives detector or search me.”