They wouldn't need to be in people's cars. You could just bounce a laser off the car windshield to hear what's going on inside.

This strikes me as a great tactic, as I can imagine people that are preparing to lie to the authorities may be corroborating their story as they approach the gate.

What surprises me is that the border patrol officer would tip this hand over such a minor thing.

Fair enough. I was not really aware those technologies are/might be used this intrusively.

Would that work on a car, considering it's vibrating from the engine running?

Check this out: https://en.wikipedia.org/wiki/SpectraLayers http://vimeo.com/32731023#at=0

Ok that's pretty damn cool, I didn't know that was possible. But the car vibrations might still be too much noise, and I don't know if it's possible to automatically remove the noise like that.