If Marcus was only tested once, after one game, and tested positive, then the probability of him being doped are 100% as far as I see it. His chances of being detected, or maybe the chances of him being tested after any of one the three games are maybe 33%. I'm not a mathematician, so don't know how to calculate the odds that he only ever chose to dope on the exact day that the league chose to test him, but I am enough of a cynic to speculate that those odds would be around 0%