Marijuana is tricky, no one gets the testing right. Even pupil tests are subject to abuse. Guys I know did their baseline tests in a suboptimal state so they could still get on site when zonked.I'll also add that the argument for current drug testing criteria in the work place is actually a lot weaker than the argument for a requirement for vaccination against an infectious disease.
Having a couple of spliffs a week prior to starting back at work may trip you up in a random drug test and cost you your job even though those sweet, sweet jazz cigarettes will have had no effect on you or your colleagues ability to perform work.
Whereas introducing a case of the lurgy that could potentially wipe out half the workforce for a couple of weeks (or worse), is something that should probably be frowned upon at least.