There is no such exception allowed in the Constitution. And if a case is made that they're not legally in the US yet, then by the same logic, they should not be subject to all the same laws of the US yet.

Even Wikipedia spells out that invasive searches require "reasonable suspicion." So we return to the core question... suspicion of what? Suspicion is not a crime. https://en.wikipedia.org/wiki/Border_search_exception