[Tangurena2]

The NK workers tend to be locked up in dorms in different countries. They can't go to drug tests because their handlers won't let them. Likewise, they're not actually in the country they say they are in.

Additionally, drug testing locations examine government IDs all the time, so the NK workers are not likely to actually have one that matches the name they are working under. Otherwise, people who use illegal drugs could hire "clean" family or friends to give the hair/urine/blood required for the tests.

People who have escaped from Scientology report being treated similarly - guarded at all times when they leave their compounds, ID documents seized.