If you have celiac disease, it's important to be aware of places that hidden and trace gluten can hide. Small amounts of gluten can be enough to cause damage to your small intestines, antibodies to be produced, and symptoms to be experienced.
Gluten can be found in unexpected places, often as a result of processing and packaging, but also as ingredients that are not clearly identified on labels. In the US, wheat must be called out on foods, but barley and rye (and oats which may be cross contaminated unless certified GF) do not.