I can only imagine this being a concern if you are planning to bury your seed phrase outside somewhere. If you store it inside your house or other building, inside a safe, in a safe deposit box, etc., then metal detectors become useless due to the presence of copious amounts of metal in the close vicinity. Stainless steel also happens to be one of the hardest metals to accurately detect using a metal detectors since it displays very low levels of magnetic permeability.