Hemp and Marijuana are just different varieties of the same plant genus (Cannabis sativa). THC (delta-9-tetrahydrocannabinol) is present in all hemp varities to some extent - about 0.3% in rope hemp and 6-8% in plants grown for its psychoactive potential (cultivation and selective breeding techniques can produce up to 15% or more).
Hemp seed and hemp seed oil (the food products) can contain 0.025% - 0.3% THC levels, depending on growing conditions and processing. Since these fall below the FDA labeling requirements (only 0.500% or greater of anything must be listed) the THC content may not be listed.
Like with the poppy seeds on your rolls or bagels (which register as morphine, codeine, opium, heroine) the hemp seeds can show up on a drug test as marijuana use (after all THC is THC). If you pass or fail depends on the "standards" they are using ... how many ppm/ppb they find acceptable.
Hemp seed has some good nutritional values. It's unfortunate that legislation keeps further research and growth into it's potental as a food product on the "no-no" list.