mediumone
Contributor
Secondary Security Screening Selection
It's a "random" TSA thing which makes travel a sh1tshow. Once it's on your ticket, they will check you mercilessly. I've had it on about 4 flights in 25 years of intensive travel. You get stopped at every possible checkpoint and your toothpaste will be x-rayed. This pic is from Wiki.
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And you might get the ssss even with pre-check?