Throat Mic?
“Throat mic. Puts me through the speakers.”
I kept wondering about this and had a look around. Each and every throat microphone I could find - be it WWII or new - is visible from the outside because it is not placed in the throat but receives the sound waves from the outside. Which means that Eurus’s mic would have been visible for Sherlock - and for us. She may have hidden the earpieces under her hair but not the transducers, i.e. the sensors picking up speech.
https://en.wikipedia.org/wiki/Throat_microphone
One more impossible thing in an impossible episode.















