In reading the first few sentences I was going to ask if you pick it up in 3F OG, then you said you did, so I think I know what you are talking about. I pick it up, although just slightly, and have associated it with yeast autolysis. Since sour beers typically ferment and age in the same vessel, autolysis is inevitable. However, because they have brett in them, the autolyzed off flavors are eaten up and converted into more pleasant flavors. With some sours (and 3F OG), I pick up a sort of phenolic flavor. Its very slight, and I actually find it pleasant at that level. I'm guessing what you are picking up is the aromatic side of this?