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."
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.