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It's possible to have SOME in your system from being around it. All urinalysis tests have a cutoff value though. The test is actually looking for the metabolites (what your body turns dope in to) in your urine rather than the drug itself.
What it comes down to is how sensitive the test, and what the cutoff threshold is.

Military drug screening is set with such high threshold that there is no doubt or possible arguement about what one did or didn't do. The samples are also tested using three different methods, all of which have to return positive for the result to be a valid positive result.

Do you have any more info about the particular lab and screening method? Otherwise, I would suspect that your friend is full o' crap, or got so hammered that he doesn't remember exactly what went down.

Micah