Update: A day after this story was published, Air Canada reinstated Nina Chung's ticket. In an email, the airline's director of customer relations, Michael Tremblay, wrote "the appropriate people are trying to determine what happened so we can prevent it from happening again." The airline industry regulator, the Canadian Transportation Agency, says it's aware of other cancellations and is "following the matter closely."

A number of Air Canada customers have had their tickets cancelled without warning — the latest is a Vancouver woman who only learned her ticket had been refunded when she checked her credit card statement.

Nina Chung, 35, says she received no notice from Air Canada.

Even more confusing, her booking on the airline's website still states the ticket is confirmed. But airline representatives have told Chung the site is wrong — she doesn't have a ticket.

Nina Chung's Air Canada account says her booking is confirmed, but airline reps told her it had been cancelled. (Dillon Hodgin/CBC)

"There could be so many people that are affected," said Chung. "It's just hard to judge how many."

Gabor Lukacs, an air passenger rights advocate, said the issue appears to stem from changes to Air Canada's fraud detection system, which is falsely flagging valid credit cards.

Lukacs said he has received "a number of complaints" from Air Canada passengers in recent weeks, all involving surprise cancellations of bookings.

"There is no doubt there is something fundamentally wrong here with the system," said Lukacs. "A fraud detection system that has so many false positives is obviously faulty."

Air Canada: 'sincerely regret inconveniences'

Air Canada spokesperson Angela Mah denied there has been a change "in the occurrences or the likelihood of credit card holders being flagged," but appeared to confirm the airline's online security system has been undergoing changes.

"We're optimizing our anomaly detection and prevention tools and security systems constantly, as credit card fraud costs everyone," Mah wrote in an email to CBC News.

"We changed our notification system in April to let those customers know as soon as possible [about cancellations] in order to allow them as much time as possible in case they need to use another card for payment," Mah acknowledged.

"We sincerely regret inconveniences which may affect our customers …we will be happy to look into any customer situations which do not appear to follow our processes of notification …"

Mah claimed the Chungs were notified by email of the cancellation of their ticket. The Chungs say the only emails they received involved Air Canada confirming their itinerary.

Chung is worried other Air Canada customers don't realize their tickets may have been cancelled (Dillon Hodgin/CBC)

Lukacs said Air Canada's response is simply not good enough.

"The biggest problem from my perspective is that Air Canada does not contact affected passengers and seek further information or clarification from them," said the Halifax-based activist.

'Air Canada must compensate passengers'

Lukacs maintained that Air Canada cannot unilaterally cancel a ticket unless there is proven fraud.

He said that by doing so, the airline is violating its own terms and conditions, as well as domestic tariff [general rules] that govern the transportation of passengers.

"[When] Air Canada ... forces passengers to rebook flights, Air Canada must compensate passengers both for the fare difference, and if the passenger is not able to travel on the original flight, also must pay denied boarding compensation."

Lukacs said compensation could be as high as $800, "because passengers did have a valid ticket which Air Canada mistakenly cancelled."

There have been several similar ticket cancellations across the country in the past several months, but they were reported as isolated cases.

Pattern of cancellations

A CBC News investigation, however, shows a pattern: Credit card purchases made on the Air Canada website were cancelled without notice.

All passengers involved weren't told by the airline, most only discovering they had no ticket when they arrived for their flights.

That happened to a Calgary man and a Toronto woman in January, a New Brunswick woman, a Newfoundland couple in March, a Saskatoon man, and a Canadian living in San Francisco, in May.

Chung's husband, Wilkins Chung, says phone and online chats with Air Canada reps got him nowhere. (Dillon Hodgin/CBC)

Most were required to buy new tickets at much higher prices at the airport and then had to fight the airline for refunds.

"Air Canada's conduct is egregious, and it reflects a complete disregard for the rights of passengers," said Lukacs. "It's the typical corporate arrogance."

Chung said she only discovered the ticket cancellation and refund by accident, when she checked her MasterCard statement weeks after purchasing a seat sale ticket to Hong Kong. She charged the ticket May 10, was refunded May 15 and discovered the charge back May 29.

She was stunned, because she has no credit card issues, her account is nowhere near her credit limit or expiry date, and she regularly pays off her card.

In fact, Chung — an Aeroplan member and frequent flyer — had used the same credit card and personal information to fly with Air Canada in January.

"I called the credit card company just to make sure there was no kind of fraud or something," said Chung. "But they said it wasn't their fault.They said it was Air Canada's fault … they said maybe you should call [the airline] to figure out why there was a refund."

That's when her husband, Wilkins Chung, got involved — and he said the runaround began.

'Did not meet ... validity checks'

Chung provided the CBC with screen captures of his online chat with an airline rep via Air Canada's Facebook page.

"Our reservation agents wouldn't be able to assist you over the phone, unfortunately," the anonymous agent wrote. "Air Canada reviews all ticket transactions for validity, and it appears your transaction did not meet all the validity checks. For further assistance, you would need to complete the transaction face to face at the airport."

Wilkins and Nina Chung are angry they're being told to go to an Air Canada ticket agent at Vancouver International airport — a two hour round trip from their home — with no guarantee the issue will be resolved.

His advice to anyone who has charged a ticket on the Air Canada website?

"I wouldn't even wait for a [credit card] statement. Check online to see if there is a refund for the same amount from Air Canada and then contact them, because there is a problem in their system."