I would suspect that as well, but it is still an important question. I would think that there would be rules regarding cavity searches that clearly state things like “If an x-ray shows no sign of foreign objects inside the suspect, then this is sufficient evidence that the suspect is not carrying anything.”
Trying to search for information regarding this has mostly led to news articles about these recent cases, but for crossing the border it appears that “strip searches, cavity searches, involuntary X-rays and other particularly invasive investigative techniques” “require reasonable suspicion, but no warrant” ( https://ssd.eff.org/your-computer/govt/warrantless )
