I worked in radiology for over a decade as an MRI tech. MRI scanners are extremely sensitive to any metal as it disturbs the magnetic field and causes artifacts on diagnostic images we would acquire. I have personally came across quite a few cases of unexplained metal foreign objects in patients that had no idea what or how they would be there.
No previous surgery, no trauma.
The bizarre thing is that typically a foreign body lodged in human flesh would leave a track mark easily visible on MRI in the form of a scar tissue. It is as if these patients were either born with them or they were implanted by some very unconventional means. There are a few "people" tracking these cases...
Some have been documented and researched and let's just say we are not familiar with the types of alloys that were surgically extracted.
