The US has been learning how to ‘switch’ genes on & off of small sea life for years, and are finally ready to release their genetically modified critters into the wild as tiny spies.
What they learned was microbes will respond in a specific way when their environment has a specific type of atom around it. Sometimes this response means ‘getting rid of an electron.’
The act of giving off an electron is a tell-all sign that something metal is in the vicinity of the microbe. Get BILLIONS of these things to register metal, and you can gather the general size & velocity of it, giving away enemy positions no matter the depth.