It can also come down to contamination in regards to barley, rye and oats which is why those who are avoiding wheat tend to go with a gluten free diet. Oats are very suspect and unless they say gluten free are almost guaranteed to be contaminated with wheat.
Corn is a harder ingredient to remove from the diet because it is a non-top 8 allergen so it doesn't have to be clearly labeled. So while it's easy to avoid things like corn syrup, it's harder to determine if the preserving agents are corn based and honestly CSR's are generally clueless when you call. There are some extensive lists on the web of food that are corn free - I attempted to work with them and remove them based on other allergens we were avoiding.
Which brings me to this. My son was found to be allergic to corn (and a host of other things) based on testing alone. However, he has never had a reaction to it (short of being small in stature so I had an idiot allergist try to convince me he was allergic because testing showed it to be true). True corn allergies are rare, I am not saying non-existent, just rare. So if your allergist is going based on testing alone for any of your food based allergens without your body having an allergenic response, it might be time to find a new doctor. If this is a naturopath, promise me you will run screaming.