The letters stand for Secondary Security Screening Selection and if they appear on your boarding pass, it means your name is on a highly classified roster - the Selectee List. Once they come to pass through security, they'll likely be subject to “enhanced” pat-downs while their luggage might be inspected by hand.
If SSSS appears on your boarding pass regularly, you can apply to the Department of Homeland Security (DHS) Traveler Redress Inquiry Program (TRIP). That program may remove you from whatever mysterious watch list you may be on — if you're not a security threat, that is.
TSA doesn't publish specific guidelines on why certain passengers get SSSS markings on their boarding passes. Anecdotally, passengers with unusual travel patterns, one-way international flights and travelers to certain countries get it the most. It is also possible that TSA assigns SSSS markings randomly.
You can find flight numbers on your boarding pass, flight ticket, or booking confirmation. One of the most direct ways to locate your flight is to see it near the top of your physical or digital ticket.
A Secondary Security Screening Selection or SSSS designation on your boarding pass means that you've been picked for a secondary security screening. The Transportation Security Administration (TSA) may select passengers for SSSS on flights from, to and within the U.S.
It all depends on the country and the airport. Some metal detectors and scanners are set to give a false positive signal at random intervals, leading to a personal search, but in many instances what are declared to be random searches are not that at all.
Why Does the TSA Swab Hands? The Transportation Security Administration randomly swabs passengers' hands at security checkpoints and airport gates to test them for traces of explosives. The TSA swabs are analyzed for nitroglycerin, nitrates, glycerin, or other chemicals.
TSA protects transportation systems: they scan your luggage, ensure your ticket is valid, and provide airport security. The only reason they might be interested in your phone is if it looks suspicious.