Battery powered, wearable, non line of sight, short range metal detector?
RFID is ideal for that. There are very small glass encapsulated tags which are typically for injecting into pets. Transceivers are pretty cheap too but I'm not sure you'd want to be installing one in someone's mouth.

A pendant would probably be a bit small for the necessary coil but you could loop the coil around the subject's neck, I guess.