Whenever I hear that I always wonder how the person making that claim knows. If you don’t have a metal detector how do you know there is no metal?
To use your analogy. Who said there is no metal? You only need a metal detector once. Once you know something exists then you have no reason to keep proving it. But if you go out looking for metal and then complain that you got cut picking it up, despite being told over and over that you shouldn't, then the problem lies with you.
Metal can be sharp. If it looks sharp, possibly rusty and likely to give you an infection, don't touch it. If you get infected, that's on you.