I don't know how the ingredient laws differ from the UK and US, so I can only speak from my experience in the US.
There are products in the US that indeed contain trace gluten, but it isn't listed on the label. For example, some brands of frozen french fries where they are made of potato, but the conveyor belt in the manufacturing plant may have flour on it, to keep the potato from sticking. It isn't required to be listed as an ingredient. There were other things like that- boxes of raisins sometimes? The problem is, of course, that a product may be fine for your daughter for years, and then the manufacturer pulls a stunt like this out of the blue, and you are none the wiser until she reacts.
Then as mechurchlady pointed out, there's cross-contamination. And a manufacturing plant that for years did gluten-free may start a new line with something containing gluten. My DH works in food manufacturing, and I do know that in the case of where he works, they are very very picky about cleaning the lines between products. But I don't know anyone that is really gluten-free that would eat the products that they manufacture, simply because so many of their products do contain allergens that it's hard to be sure it's 100% no doubt not cross-contaminated. If the brand you're buying is coming from a plant that specializes in gluten-free products, that's your best bet, cause you know the allergen isn't even coming into the plant.
It also seems that recently, food manufacturers have been cutting corners as much as possible, which sometimes means using more filler, which is baaaaaddd for allergy-people. DH was just telling me a few days ago that the chicken nuggets they manufacture, they are starting to add soy to them for filler-- not that we ate them in the first place because they have a flour coating-- but they didn't contain soy before, the package hasn't really changed, and if someone wasn't paying attention they probably wouldn't notice. Actually, they're hoping people don't notice, that most people will continue to buy the product without realizing they're getting less real chicken and more filler, so it's not like they're making a big effort to point out the new recipe.
I had the same thing happen about a month ago with Rice Slices, I went to the health food store, and was all the way to the check-out when I noticed the package said "Improved Recipe" in the corner, checked the ingredient list, and found out their "improved" was that they added casein to them. UGH!!! What's the point of making rice cheese if you're going to put casein in it? And DS was totally addicted to the rice slices. The health food store manager hadn't even noticed the change.
I'd call the cereal manufacturer and find out. You might have to get transferred around a bit to find someone who knows anything. And switch her cereal to something else that is manufactured by a specialty company. The big thing I've noticed with cereal is Malt Extract, it seems to be in almost every cereal, even the ones you'd think would be gluten-free. I don't understand why a cereal like Corn Chex needs to have malt extract. And Rice Krispies. We've purchased the gluten-free Rice Krispies and they taste exactly like the Kellogg's ones, so why does Kellogg's put malt extract on theirs? It isn't adding anything to the flavor, but it's taking away some customers.
