More stringent or random testing can foil detoxification plans. Products added to urine. Coloring or bleaching hair. Difficult to conceal. Smell or color of urine could be affected.
Lab looks for specific tampering products.
Other body hair is tested. Well it does only because everybody thinks that a person who smokes weed is a druggie who is worth less which is not true. They have no probable cause for most of the people that have been tested. Fortuanely for me I was clean as a whistle but some have come back with impossible findings. People have basically been harassed, embarrassed by management level employees making certain peoples test results public knowledge among other employees.