It's no news to people with gluten sensitivity; that white flour was crushed from the wheat plant not from the stalk of the mysterious "white". (Regardless of puzzled wait staff's insistence to the contrary)
But if it comes from wheat, not corn, then where, exactly, is the corn?
The corn can come from a couple of places. For one, when you're dealing with grains and milling equipment, there is a high risk of cross contamination. If you've ever turned on a mixer too early, you know how easily flour gets airborne. Those particles can settle anywhere and everywhere, and they are hard to completely eradicate. Cornstarch is the same way, and if the milling equipment is used to process corn; well, there's bound to be a bit of cornstarch in your flour regardless of what the label reads.
Another distinct possibility falls under the heading of "enriched". The goal of the FDA was to make sure that when they stripped away all of the nutrition found in the hull of the wheat (you know, the "whole" in whole wheat, the part that makes it chewy.) they put some back. I've heard it described as cashing in a dollar for a shiny quarter. Sure, it's shiny and fits nicely into the newspaper stand. But, it's only a quarter.
Anyways...Those vitamins that replenish some of the lost nutrients have to come from somewhere. And some of them are derived from corn. They also need to have someway of being transferred from point A to point B and nicely measured out into the flour. For many vitamins, the best carrier seems to be with corn derivatives such as dextrose and corn starch.
So, when you see the words "enriched" be aware: Those aren't just vitamins. They're corn in disguise!