Yeah your growing hydro, they tend to grow much faster than soil because of the constant supply of nutes.
Again, I don't' think it has to do with needing a flush but can't hurt. The only time I gave my ladies a flush was when I switched them to 12's, gave them a 24 hr period of fresh water.
If your gonna flush do 24 hour of fresh water, then add new water and nutes. Don't deprive them too long of their food, they are dependent on it now.
If your sure its not pests then it may be in the genes, I had 2 plants that would randomly have a spot or two come up or some discoloring. I knew it was nothing because it was only on 1-2 leaves and the rest of the plants were just fine. Sometimes you just have to wait these things out, last thing you wanna do is start changing a bunch of stuff and make the problem worse than it was.