There’s these pointless immigration checkpoints out there that serve no real purpose. The agents stop people and ask them if they’re American citizens. Apparently the idea that someone from another country might just lie is something that no one considered. All these stops really do is harass and annoy American citizens going about their daily lives. I saw them in action myself once, and was appalled.
It’s important to know your rights in these situations. When they ask if you’re a citizen, you don’t have to answer. They don’t have any probable cause to detain you, so you’re free to go (though it doesn’t hurt to confirm that). Certainly don’t let them look in your car or trunk, no matter how nicely they ask. You might think you have nothing to hide, but if you’ve ever had someone in your backseat, you have no way of knowing if, say, a joint fell out of their pocket.
These checkpoints serve no real purpose and need to go away. Check this video for further information, and remember: always be polite.