"Q: Can hair be affected by cross-reacting substances such as over-the-counter medications?
A: Enzyme-immunoassay antibodies (EIA), similar to those used to test urine, are used for the initial drug screening in hair; therefore the potential for substances such as over-the-counter medications to cause a false positive screening result does exist. To eliminate the possibility of reporting a false-positive result due to cross-reactivity, our laboratory performs Gas Chromatography Mass Spectrometry (GC/MS) for all specimens that screen positive. "
link
This
site has a good discussion of threshold values and false positives, mostly with respect to urine testing (which is similar to testing hair, but with some sampling differences). Under barbituates, naproxen and ibuprofen are listed as false positives. Under opiates, poppy seeds are list as one of a number of substances giving a false positive.
With respect to threshold values of opiates, the values of the screen and the confirmatory test are identical, 2000 nanograms per milliliter. Therefore, if the authorities used the same two threshold values (by no means a certainty), then a positive screen result followed by a negative confirmatory result could not possibly be explained on the basis of not having enough material. The negative confirmatory result would have to be the result of the presence of an interfering substance. Therefore, I think that Barbie's report is very questionable.