I had MRI a lot of times (I have MS). Every single time as we are walking to the machines, the nurse / technician / whoever asks me a couple of questions (which I have to fill before going in as well). Then I have to take off my clothes in the changing room. They would have never missed it. And no one can just simply walk into the corridor (there is a door) that has the door to the MRI machine. Even if they do so, they would be noticed immediately.
Doesn't even need to be metal: they make sure you aren't touching skin-to-skin anywhere while you are in the machine (for example, don't put your hands together) in order to avoid induced current loop burns.
1) ah yes, 5kg if gold in this guy's neck has to be real!
2) a non-magnetic metallic mass that large will still likely screw up the image, if not the machine.
I seems like an easy mistake to make. The imaging was done, per the article "His wife told local media she had called him into the MRI room after her scan" and so the technician could have looked at it being gold colored and didn't apply critical thinking to presume it wasn't real. There was no concern about screwing up the image.
I wonder if the chain was gold colored and so the people assumed it was gold and safe.