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.
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.
