I was recently involved in a part 91 incident (no fault of mine but that is not the question) That evening there was no questions asked by the FAA or mgmt about taking a test, several days later I was told I should have filled the "cup" (which is in the airplane but I didn't know that) and taken a voluntary sample for my own protection. How could that protect me? I would be innocent of alcohol/drug use, but by not taking it they have assumed I used or could have used alcohol/drugs? Sounds like guilty until proven innocent to me.