In a perfect world, there would be no contaminants in your food, and in a slightly less-than-perfect world, any foreign piece of metal in your product would be spherical in shape, making it easy to spot in a conventional, single-planar field metal detector. However, unless a ball bearing fell into your product, you’re more likely to find metal flakes that—depending upon their position in your product—may completely evade your metal detector, which is better at detecting 3D objects than practically 2D flakes of metal positioned perpendicular to the sensor’s single electromagnetic field.
Why? A flat metal flake appears at best as a single line in a 2D plane (or a magnetic detector field), and when perpendicular to the plane (field), as a single point, which is extremely difficult to detect. For a better illustration, read “Flatland” by Edwin A. Abbott for a story in perception of points, lines and planes in single-, two- or three-dimensional spaces.