Also, the border is 100 miles from the Mexico/Canada borders and 100 miles from the shore. So, if you're concerned it's not when you're entering or leaving the country. It's any time you're in LA, NY, DC, SF, Huston or Detroit. Or any of the other thousands of miles of border.
Common misconception: If you have crossed the boarder, then within 100 miles of it they can search you.
Of course, how you prove you didn't cross the boarder is an open question. But you can in fact refuse the search on that claim. I suppose they may detain you then.
I think you're looking for the phrase `probable cause`. If the officer has probable cause to think you crossed the border, they can search you. I'm not a lawyer, but i think they can search you anyway. It just won't be admissible in court.
edit
to clarify, US law pushes police right up to the edge. there is a preference for false positives rather than false negatives. it's more important to catch all of the criminals than it is to inconvenience some innocent people. The risk of letting one criminal go is much more than cost of detaining a doctor for a couple of hours.
Now, we're in this weird time where that doctor can have 20 years of hippa protected medical records in their pocket that they might be forced to disclose. Historically, that doctor may have some records in a briefcase, but not tens of thousands.
They can even nab American citizens for drug possession:
"One of the people arrested was a U.S. citizen who fled the checkpoint and led the police on a five-mile chase. The unnamed man was arrested and charged with three felonies, including reckless driving, possessing a controlled substance, and endangering the welfare of a minor."
