We saw him doing the drugs at the bar for sure, hard & real drugs. And we know for a while afterwards (after the "shootout" and hero bit) that he stays with the force for a while. So having re-done drugs weren't an issue for him at that time, at least. I'm just saying that I can see him, after leaving the force for whatever reason he left, pretending to be a loser who just works at a bar and drinks on his days off, in order to again be under a kind of undercover. Buying bottles of booze but pouring them down the drain, to make for being plausibly "off the grid" drunk biker-type but still staying sharp and investigating on the sly. Unbeknownst to Woody Harrelson's character, of course. I'm just imagining the cliffhanger of this season being Woody and Matthew teaming back up to find the killer, with the big reveal being that Matthew has a storage bay full of evidence he's already collected. And that he's been "Keyser Söze"ing the interviewing cops (and the viewers) for most of the season. Which might make Matthew's character the "TRUE Detective" of the show. Just a thought.