The 2016 NFL Draft prospects seem to understand the deal. When you KNOW you're going to be drug tested, don't have drugs in your system (or at least don't have drugs that can be detected in your system). It seems obvious but every year at the Combine a player tests positive. This year, however, no player did, according to ESPN's Adam Schefter.
For the first time in recent memory, NFL has notified all 32 teams that no player tested positive at February combine, per league sources.— Adam Schefter (@AdamSchefter) April 25, 2016
Testing positive at the Combine shows you either have a problem or you're stupid because you knew you were getting tested. Every year there is a player that tests positive and is made to look dumb ... except this year!
These are still kids so they're going to make mistakes along the way. They just didn't make this obvious mistake.