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They do not typically test for antibiotics when doing a military drug test. While they can certainly find them (the test iz that sophisticated), they don't bother testing for them unless they are specifically asked to do so.
They are testing for illicit drugs, like marijuana or methamphetamine.
While I'm not aware of any antibiotics that would cause a false positive on a preliminary test, if it did there would still be the confirmation test which is much more scientifically sound in knowing exactly what the drug is...and if that happened, they'd determine it was an antibiotic and not report it as a failure.
That may be the case, but that is only true for the initial test. Not for the confirmatory Gas chromatography–mass spectrometry test.
That test determines what the metabolite actually is. When there have been errors found, it is with the handling of the samples, not the machine giving false positives.