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you could conceivably be a citizen, but CBP/ICE think you might be here illegally, so they can stop you without a warrant.That's terrible advice, please stop giving terrible advice like that. ICE need to have probable cause (about you specifically, by name, not just your street or zipcode or "people of your ethnicity/appearance" or "people in that store/restaurant/parking lot"), beyond a verbal "uhh we think you're here illegally". You have the right to ask them why. And the 4th Amdt still exists, for all people, so no they can't stop you in your vehicle or inside your own house without probable cause and a [signed, valid] warrant [naming you specifically, and signed by a judge].
> How are you going to prove you're a citizen?
If you're a citizen, you can simply state to them you're a citizen (better when you're on bodycam; and repeat it (on bodycam) to any other ICE officials you interact with, and make mental note of their names or else descriptions so you can subsequently identify them). Pragmatically, you could also identify yourself as such to any bystanders or anyone videoing things.
"U.S. citizens do not have to carry proof of citizenship if they are in the U.S." [0] Read that again and again. Ok?
For sure ICE could knowingly violate a citizen's rights, detain them unlawfully (for hours or days), beat them up, send them to a detention center, even (in very rare cases) deport them. That becomes a political as much as legal matter. If you have constitutional/legal rights and due-process but refuse to stand up for them, do those rights functionally cease to exist? Do they magically get restored in 2029(/2033)? Time to stop being complacent.
(Obviously noncitizens do not have all the above protections, and need to be more careful.)
In the meantime, to inform yourself about your rights, and to get advice how to deal with ICE, both for citizens, GC holders, visa holders, visitors and undocumented people, see: https://www.aclu.org/ or other webinars. But, stop speculating or spreading misinformation. If you don't know, ask here.
[0]: https://www.aclu.org/know-your-rights/immigrants-rights