'Natural flavor' can, literally be anything. It can be made in laboratories, like chemicals or gasses, or taken from animals, or boiled from bats.
Here is the actual definition from the federal regs, “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.”
That was a mouthful. No wonder we get full when foods and drinks have natural flavors. The natural flavoring in orange juice could actually be pigs feet, as long as they were ground, distilled, etc. The good news is that anything not listed in the reg above must be listed as 'artificial'. I feel so much better.
Artificial flavorings are safety tested and natural flavorings are not. This is our government at its finest. For the curious, there are more FDA definitions here LINK
if you have children in school and they are looking for jobs for the future, tell them to become a 'flavorist'. It is more fun than being just a chemist and pays well, especially for flavorists in the perfume industry. The need for flavorists is growing. Luckily, bacon still does not contain any natural or artificial ingredients.