Gluten can turn up in foods where you would not expect to find it, licorice for example. But that’s not the same thing as hidden gluten. Improvements in labeling laws for both allergens and gluten have made it much harder for a product to contain gluten without listing it on the label. In particular, new gluten-free labeling rules in the United States, now address gluten that might previously been hidden because it came from cross-contamination and not from an ingredient intentionally used in a food.
Now, a product labeled gluten free has to contain less than 20 parts per million of gluten, including any that unintentionally gets into food from field to table. That takes care of the biggest source of what could legitimately be called hidden gluten in food. If a product labeled gluten free is found to exceed the gluten standard, it is considered misbranded and subject to recall and other regulatory action.
Even if a product is not labeled gluten free, wheat always has to appear on the label of any food in which it is used. Rye is not used very often in ways that would not require it to be labeled. Barley is a little trickier because allergen labeling laws do not require it to appear on ingredient lists, but typically it does show up as malt, malt flavoring, malt extract or barley malt extract.