Hidden corn is my specialty

Vitamins contain corn. Tea bags can contain corn. Milk cartons can contain corn. Licking envelopes exposes you to corn. Xanthan gum is often grown on corn (It uses the aspergillus mold, if I understand correctly, so it is also possible to be allergic to the gum itself...in which case you'd also be allergic to corn which is nearly always contaminated with the safe to consume variety of aspergillus.) Citric acid is also nearly always a corn byproduct. I've not found an alternative milk that doesn't contain corn or gluten, supposedly there's a pure soy one...but...that won't help you :P Oh, and flours can be cross contaminated with corn starch. They often use it to clean the machinery.

cornallergens.com has a list of corn derivatives.
And here's a summary of potential uses for corn. http://www.iowacorn.org/cornuse/cornuse_2.html