Oh wow, I had no idea they scan you after re-entering the country. I'm flying to Chicago O'Hare from London Gatwick on Dec 2, and now I'm concerned about this. Does anyone know precisely what my rights are as a US citizen upon re-entering the country to avoid all of this ridiculousness? I know that Customs has some Constitutional power for searches and so on (and there's that insane 100-mile "border" where people can…
You're required to present proof of citizenship and allow a "routine" search of your belongings for contraband. A "non-routine" search requires them to have a reasonable suspicion about you specifically. My own reading is that a backscatter scan or s search of your body would count as non-routine, but of course a court may disagree.