There's a large gap between audible and detectable. A transmitter could transmit a weak signal in a specific pattern on a number of arbitrary preselected frequ

There's a large gap between audible and detectable. A transmitter could transmit a weak signal in a specific pattern on a number of arbitrary preselected frequencies. They would more-or-less seem like noise lost in the noise floor. Only when you specifically filter for the specific set of frequencies can you discern the pattern that was put there that could not appear by chance. You don't even need to use the same frequencies throughout the transmission. As long as the schedule is decided up-front it can serve like a one-time pad for encryption.