Firstly you should store your seed phrase yourself. Those stuff about X-ray detecting your seed through metal plates can only be done if you don't keep your seeds private. Basically no one will go looking for something where they didn't keep them and based off of that condition no one will attempt to make use of detectors of any kind in search of your seed if they are not aware that you have a seed there.
Security and privacy actually go hand in hand and you can't separate them. As much as you are actually trying to make sure your Seed is safe you should also make sure that your seed is private.