The city you’re flying from (Los Angeles, Chicago, New York), your clothing (a hoodie), and even eagerness to get your bags at baggage claim are suspicious and can get you stopped. So can looking like someone with those characteristics that the police notice while following someone else. You’d be surprised how often you’re being watched at the airport, and that the government can stop you — and take your stuff.
Last year it was revealed that the TSA was assigning agents to follow ordinary Americans and record their behavior. That behavior was exposed and shamed, both because it was creepy and for no legitimate purpose. But creepy violations of civil liberties don’t just stem from the war on terror, they’re still a feature of the war on drugs as well.