Air travel is designed to move millions of people quickly and efficiently, but for some passengers, the journey includes an unexpected delay marked by four letters on their boarding pass: SSSS. SSSS stands for Secondary Security Screening Selection, a designation used by the Transportation Security Administration (TSA) to indicate that a traveler has been selected for additional screening. Spotting this code—usually printed near the gate or seat number—means the airport experience will be more detailed and time-consuming than usual.

The SSSS designation is part of the U.S. government’s Secure Flight program, a pre-screening system that reviews passenger information provided during booking, including name, date of birth, and gender. This data is checked against government watchlists to identify potential risks and prevent mistaken identity. Importantly, most travelers flagged with SSSS have done nothing wrong. In many cases, passengers are chosen randomly to maintain unpredictability in airport security.....