Hm, maybe. Radios mix signals with a local oscillator to get an exact intermediate frequency. If the local oscillator isn’t perfectly shielded, it’s also a transmitter that tells which frequency the radio is tuned to. (Used in WWII, I believe, to detect citizens listening to the wrong stations.)
Advertisers might have used that even before electronic signs. Knowing the demographics of your target audience along a route would be gold to them.
