The same way metal detectors work in other places, like airports: if you set off the detector for a valid medical reason, you should be prepared to show the card your doctor gave you attesting to this fact.

Because people who get a doctor to implant something in their ear will have so much trouble getting such a card?

Implanted bluetooth devices is an edge case; so much so that it made the news. The normal case is people hiding devices in their clothes.

Anyway, I'm not sure whether it's considered unethical to help a patient electively implant a bluetooth receiver in their bodies, but falsely signing a medical release card probably is.

"If you say the metal detector is picking up a piece of shrapnel from an accident while young, how can they really disprove that?"

X-Rays? MRI? Or requesting medical documents about it? (can all be done afterwards)

Sure it's possible, but that's asking a lot just to take a test.

I do not think, many people have metal from accidents in their body. So it would be a rare exception.