Air Canada continues to investigate how crew members could have disembarked from a plane without noticing a sleeping passenger who was still on board, an incident experts say raises serious security issues.

After flying from Quebec City to Toronto’s Pearson International Airport on June 9, Tiffani Louise O’Brien woke up from a mid-flight nap to realize she was completely alone, locked inside the pitch-black aircraft.

“[I was] still in my same seat, 32A, which is about middle of the plane and it was completely pitch-black,” she explained to CTV News Winnipeg.

“I thought this is a nightmare, this is not happening, I am having a bad dream.”

O’Brien said she woke up to several text messages from a friend who had dropped her off at the airport in Quebec City, concerned that she had yet to hear from her after the short flight.

After briefly calling her from the dark plane, O’Brien’s phone died. Her friend then contacted Pearson International Airport to alert ground crews to the situation.

“I said her airplane would have landed two hours ago, she texted me she just woke up,” Deanna Dale told CTV News. “I said I don’t know what’s going on but she’s at the airport.”

After searching around the dark plane, O’Brien said she made her way to the cockpit, where she attempted to use a radio to call for help, but couldn’t use the plane’s radio without power.

“I was so scared that I was going to touch something wrong, but I knew I had to do something,” she explained.

She said she eventually found a flashlight and began signalling out of the plane’s windows, hoping to attract attention.

After managing to open one of the plane’s doors, O’Brien was eventually rescued from the aircraft after being spotted by grounds crews.

Air Canada confirmed O’Brien’s story, but declined to share any additional details citing a review of the incident.

Though it remains unclear how crew members missed O’Brien before disembarking the plane, experts say it raises serious security concerns.

“If this would have happened with someone else who may have different intentions, injury could have resulted,” said Gabor Lukacs, founder of Air Passenger Rights.

“The passenger had access to the cockpit and could have, hypothetically, turned on the aircraft. That would not have been a good thing to happen, to say the very least.”

Christina Ling, a flight attendant instructor in Vancouver, said a lot of things would have to go wrong for a passenger to be left on an aircraft.

“I've never encountered this situation before,” she said. "Once the flight crew has disembarked, the aircraft is still very much a hub of business. Groomers come on, catering comes on, a maintenance crew comes on and everything is checked."

Former pilot and consultant Ross Aimer told The Canadian Press that he's never heard of such an incident in roughly four decades of aviation service.

He said the incident shows that someone with malicious intent could hide on a plane and attempt to hijack it.

O’Brien, who said she was offered a limo or hotel room by an Air Canada representative immediately after being rescued, said she’s still struggling with incident.

“It’s just a sheer sense of helplessness when you feel like you’re locked on this aircraft and you have no connection to the outside world,” she explained.