They had stored their carry-on bag containing their valuables in the overhead compartments when they boarded the flight from Malaysia to Vietnam.
But the three passengers, who were seated in the front row of the plane, later discovered their cash and credit cards missing.
The alleged case of in-flight theft occurred on a two-hour AirAsia flight from Kuala Lumpur to Ho Chi Minh City on July 3.
An employee of travel company LFG, who witnessed the incident, described it as "daylight robbery" in a video posted on its Instagram and TikTok accounts on July 5.
The 56-second clip starts with a warning to never leave cash, cards or anything important in the overhead bins on a plane.
The eyewitness said that a man seated at the back had walked to the front of the aircraft twice during the flight, taking down bags from the overhead bin and putting them back "after a while".
The three passengers later realised that their valuables had gone missing, though it is unclear when exactly the discovery was made.
In the clip, two flight attendants can be seen pulling out a wad of cash from a sock, which the eyewitness claimed the thief had stashed the money in and thrown to the back of the overhead compartment.
"Once we landed, we caught the freaking guy again trying to sneakily come out of the plane too," said the eyewitness in the video.
"We helped to do a witness report and also learned that this guy came in a group of four who always fly one-day transit flights all around the world."
[embed]https://www.tiktok.com/@lfg.travel/video/7388462586083200272[/embed]
In response to AsiaOne's queries, AirAsia confirmed the incident, which was reported to the airport authority in Vietnam upon the flight's arrival at the airport.
The Malaysian low-cost carrier added: "Our cabin crew is well-trained to handle security incidents and remains vigilant in monitoring guest behaviour.
"We also advise guests on board to safeguard their valuables and report any suspicious activities to the cabin crew immediately."
AsiaOne has reached out to LFG for more information.
Netizens urge others to safekeep valuables
Under the comments section of the posts, netizens urged passengers to always keep their money, passports and electronic devices on them.
"Always put your luggage on the cabin opposite your seat, not directly above your seat. This way, you can observe who is accessing your luggage," suggested one Instagram user.
Others chimed in with personal experiences of having items stolen on flights or witnessing thefts with similar modus operandi.
"Just happened to me on AK520 KUL-SGN on July 5," commented one netizen.
Another wrote: "The same thing happened to me two weeks ago travelling on AirAsia flight between Ho Chi Minh City and Kuala Lumpur. There is a pattern, and it needs a serious action plan from the airline and airport authorities with police."
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lim.kewei@asiaone.com