"I tried to check-in online the night before, but the website kept throwing up an error message," said Stewart.
"I didn't think much of it at the time and knew I'd be OK to check-in at the airport."
When he checked in at the airport, his pass was stamped with the code 'SSSS'.
As airline staff began walking along the queue of people, asking if they had completed certain Covid-19 requirements, Stewart told them he was unable to check in on the website.
He was taken out of the queue and over to a desk to check his details.
"After she'd punched in my passport number she said 'Oh!' and my heart started pounding when she did that. I was convinced there was a problem and I wouldn't be allowed to fly…" he recalled.
"The employee saw the panic in my eyes and explained the reason for the error when checking in was that I'd already been preselected for extra screening.
"She assured me this was nothing to worry about and was purely random. It would be a search at the London end of the journey before boarding and not in New York as I'd initially feared."
Additional screening means officials will more thoroughly search your luggage, give you a pat down and swab you for explosive residue. You may also be asked to provide extra information to prove your identity and travel plans.
The process, which was brought in by Transport Security Administration after the 9/11 terror attacks, is predominantly used for flights to and from the US.
Stewart said he did not have to queue any longer than other passengers. When his boarding group was galled at the gate and crew saw his boarding card, he was directed to a screened-off area with another passenger.
"My hand luggage was searched, my hands and pockets were swabbed – they didn't tell me what for. I was then asked to take off my shoes so they could check inside," he said.
"This all happened in the waiting area before we actually boarded, so it didn't delay me getting on the plane."
Not all experiences are this seamless. Journalist Ashlea Halpern has received an SSSS code three times while flying domestically around the US. Every time, issues began with the inability to check in online or at an airport kiosk. Halpern described the screening as "extensive" in a Condé Nast article.
Her and her partner were asked to hand over their shoes, coats and electronics before walking through a body scanner and metal detector. Then they had a full-body pat down followed by swabbing for trace explosives.
While the TSA agents were "polite and professional", the screening, which included emptying and inspecting every part of their carry on and checked luggage, almost made the pair miss their flight.
If you feel you have been subject to a "Secondary Security Screening Selection" too many times, it is recommended you file an inquiry through the Department of Homeland Security Traveler Redress Inquiry Program (TRIP). However, there are no promises that you won't receive another SSSS on your boarding pass.
