Those four letters – SSSS, for “Secondary Security Screening Selection” – can appear without warning on tickets for flights to or from the US. They don’t mean you’re guilty of anything; they mean you’ve been flagged for an intense, time-consuming security check. Your bags may be emptied, every item swabbed, electronics powered on, even your phone scrutinized in invasive ways that feel less like safety and more like exposure.

For frequent travelers, veterans, even teenagers flying alone, the code can appear again and again, dictated by opaque algorithms no one fully explains. It’s a stark reminder that modern air travel balances uneasily between protection and intrusion. You may never see SSSS on your boarding pass. But if you do, you’ll learn very quickly how small those four letters are—and how large a shadow they can cast over a simple trip.