Food and beverage manufacturers are not required by the FDA to list ingredients that fall under the FDA defined category of "Natural Flavors."
The definition of natural flavor under the Code of Federal Regulations is: “the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate, or any product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof, whose significant function in food is flavoring rather than nutritional” (21CFR101.22).
Not every processed food you eat will have meat products but shouldn't we know which?