Agreed, there is little to no testing on the road race circuit for masters runners. In the past, I won the masters title at the Chicago marathon and was 3rd masters at the US 15k road championship - both with cash payouts - and there was not only no testing done, but there was no real meaningful way to test.
When you come through the chutes at races like that, they have no idea if you are a masters runner in contention for prize money, and it takes hours for them to make the determination of who won money, and the money itself is sent well after the race. So when you are done with the race at a big race, you normally just walk off the course and are on your own. They aren't going to randomly pick the 94th place marathon finisher for drug testing a the finish line; and since the checks are not handed out at an awards ceremony that day, even after a determination of who placed is made, there is no real meaningful way to bring that person in for testing since they simply may not be around.
As a practical matter, I could have been doped to the gills and no one would ever know. (For the record, not that anyone cares about a second tier masters runner other than one occasional troll on these boards, but I have never doped, never been prescribed testosterone supplements, or anything else that would aid performance other than some mid training cycle prednisone for a foot injury. And I would be willing to pee in any cup at any time.)
That's not to say that it is impossible to implement testing, just that it does not happen for all kinds of practical reasons.
Again, none of that goes to Castille, it is just meant to facilitate discussion of what the current testing scene looks like at the masters level, at least outside of track championships meets. In Castille's favor is that he is also placing as an open athlete at many high profile events, so that probably somewhat increases the slim odds of being tested that most masters runners face.