I was in DC last week, went through many security checks and the only one that registered that I had a stress test was at the Supreme Court. They asked if I had recently had any chemo. I had the test 2 weeks before.
Interesting. I guess those detectors are really sensitive.
A relevant article:
Iowa to check for radioactive cargo (at truck weigh stations)
So I guess it's entirely believable, that they would have radiation detectors at the US-Mexico border.