Failing A Random Drug Test.

Ok question: if his girlfriend had just smoked, and then they made out, could he be ingesting THC through her saliva?

That seems more plausible than the second hand smoke as a cause. From the research I've done, it's near impossible to **** hot on a test from second hand pot smoke, for the reasons Stevo VWbusman mentioned.

As far as the hair test, that looks for metabolites of THC, not the THC itself. In other words, smoke settling on your hair will not cause a false positive on a hair test for two reasons: First, it hasn't been metabolized, and second, hair tests are looking at the stuff in the core of your hair, not the outer sheath, since that's where the metabolites of pot (and coke and heroin and meth and so on) would be present.

Another reason could be a faulty test - samples mishandled or mislabeled, testing equipment not properly cleaned, etc. Personally, I would want an immediate retest from a different testing company if I got a positive test result back.

Or it could be from brownies, or something else where some dust ("keef") got from her hands onto the food she served him right after she handled some weed.

Yeah, I was a pothead back in the day, I know a lot of potheads, and I live in Colorado, so I researched the crap out of this.

Now I know why your name is Bud...