I detected something at about 214MHz. A friend with Oticon aids says
they're at 115MHz. Different NFC (near field communications) devices
work at different frequencies, so the antenna geometry must change
accordingly. There are many published/online plans for various
directional antennas and the math is simple. If this concept were to
become a product, the antenna is the key item. It would be printed as
foil on cardboard, with dotted lines to be cut based on the brand of aid
(and therefore the operating frequency) to be detected.
Ideally, the antenna will be mounted atop a pistol-grip device or small
box that can be pointed around; the radio receiver will generate a
flashing LED when the antenna is pointing at the aid.
The radio receiver is a trivial design project for any electronics tech
(doesn't need an engineer).
I don't see more than $10 in parts for the whole shebang (a word that
people like me - over 70 - will recognize).
Tell me how to get popular support for this project and I'll convey it
to a local university's engineering department - they're always looking
for student projects.
Ike