Going through airport security can be a time-consuming experience. A necessary evil, if you will.

But there’s one code you do not want to see on your boarding pass because it’ll mean you’re destined for an extra security check, and that is SSSS.

If you’re flying into or within the US and are unfortunate enough to have the code printed on your boarding pass, you’re in for an additional screening.

It stands for Secondary Security Screening Selection and could see you forced to undergo a thorough search of your bags, extra body scans, questioning of your travel plans and a full-body pat-down.

“SSSS stands for Secondary Security Screening Selection and it appears on a passenger’s boarding pass when they’ve been selected by TSA’s Secure Flight system for enhanced security screening,” a TSA spokesman told Business Insider.

One traveller who was given the code explained what had happened to him to The Independent.

“I was flying to JFK and had only booked my flights a week earlier,” he said. “For some reason I couldn’t check in using the app as usual so I was worried something was wrong.”

After checking in at the desk he noticed the code. In the queue for the extra screening at Gatwick, he didn’t notice a particular trend amongst the people there but thought there might be more men and Asian people than there were on the plan as a whole.

His extra screening involved a swab being swept under the sole of his shoes and over his whole body which was then scanned.

He boarded the plane as normal afterwards.

According to the TSA, “Secure Flight is a risk-based passenger pre-screening program that enhances security by identifying low and high-risk passengers before they arrive at the airport by matching their names against trusted traveler lists and watchlists.”

The Secure Flight programme has been in place since 2010 and came from the laws that came into force after 9/11. However the code was seen on boarding passes before then, so it’s believed that the code was used under a previous programme too.

A TSA spokesman has said that travelers are given the SSSS code if they appear on an FBI counter-terrorism watchlist called the Selectee List, but others are assigned the code at random.