Silva apparently tested positive three weeks earlier when he was given an out-of-competition test. It's unclear why those results weren't revealed earlier. The result of his failed test will be a discipline hearing. Yahoo reports that the first offense is likely to receive a nine-month license suspension in the state of Nevada.
As for Diaz, he's a third-time offender who was previously caught with pot in his system in both 2007 and 2012. What his punishment will be remains unclear.
The UFC also released a statement on the surprising development:
On February 3, 2015, the UFC organization was notified by the Nevada State Athletic Commission that Anderson Silva tested positive for Drostanolone metabolites on his Jan. 9 out of competition drug test. UFC’s understanding is that further testing will be conducted by the Commission to confirm these preliminary results. Anderson Silva has been an amazing champion and a true ambassador of the sport of mixed martial arts and the UFC, in Brazil as well as around the world. UFC is disappointed to learn of these initial results. The UFC has a strict, consistent policy against the use of any illegal and/or performance enhancing drugs, stimulants or masking agents by its athletes.