I can't find it immediately, but I've read about something even sneakier than this. A standard broadcast station was modified such that its carrier signal was modulated by a PSK signal. The intended listener would use e.g., a PSK-31 modem to listen to the carrier signal and would be able to obtain the encoded digital data. Everyday listeners would hear the regular broadcast. The station involved _might_ have been a BBC station, but I don't recall.

You could technically just transmit data via RDS, too. Change a letter here and there and nobody would know whether that’s a decoding error or actual ciphertext. (Would need some kind of checksum or so, of course.)

@windytan did a fascinating audio clip highlighting the RDS data stream in a radio recording some while ago:

https://soundcloud.com/windytan-1/rds-mixdown