How do secret ingredients exist when surely you would have to list it in the ingredients in case someone is allergic to it?

That "secret ingredient" talk is mostly just marketing fluff to make a dish sound fancy or special. In reality, no restaurant or food company can legally hide stuff that triggers allergies. If you look at the back of a package, they’re required by law to bold or list allergens like nuts, dairy, or gluten. If they actually kept something secret that caused a reaction, they'd be looking at a massive lawsuit that would put them out of business before the week is over.

Usually, when a chef or a brand says they have a secret ingredient, it’s just a specific blend of spices or a technique they don't want to share so people keep coming back. It’s not like they’re hiding toxic chemicals or mystery allergens. It’s just the stuff that makes their food taste a bit better than the usual shop-bought junk. Don't worry too much about it, they definitely aren't risking anyone's health just to keep a recipe hush-hush.