Florida TE Aaron Hernandez fell into the fourth round of the draft, something that was inexplicable until the smoke cleared (punnery!) and the usual anonymous draft sources revealed positive marijuana tests at Florida and an admission of the pot* during team interviews.
This is something the Patriots are aware of, and can most likely manage. (See: current Patriot Randy Moss, for instance.) Given testing regimens now in both the NFL and at the college level, testing positive isn’t so much an indication of a team “catching” a player as it is an indictment of the player’s intelligence. Formally or informally, players usually have a good idea of when they will be tested, and adjust accordingly.
Which brings up the question: why test for it at all? Players who don’t use it pass an unnecessary test easily, while players who do use by and large prepare, abstain in the short term, and pass the tests. If the league wants to test players’ intelligence and planning ability, have them run fantasy leagues and tally the results. It’s cheaper than testing them for a non-addictive drug on the verge of decriminalization in some states, and far more entertaining for all concerned.
*Grandma Hall terminology.