The mysterious SSSS code means a passenger is flagged for additional—and sometimes invasive—TSA screening.
For travelers lucky enough not to know, SSSS stands for Secondary Security Screening Selection. It’s the Transportation Security Administration’s way of flagging airline passengers for “enhanced” screening. An SSSS code stamped on your boarding pass overrides pre-approval through a Trusted Traveler Program such as TSA PreCheck and can add 15 to 45 minutes (or more) to your boarding process.
This can serve as an early warning you may be in for a surprise at the TSA Screening and you should definitely be prepared to secure your devices and data. As the AMENDMENT4 User Guide details, Customs agents in most countries (including the U.S.) have the right to inspect (open, search, and copy) any data on devices you may be carrying.