I would assume it's becoming a a software war. You monitor all frequencies with SDR and try to shield as much as possible while on the offending side you try to push information on different frequencies and making it look a like like stuff that's already in the air.
It has been said that covert agencies have monitored conversations through windows by measuring the acoustic vibrations with a laser. So maybe that will work, but I have no idea how well that works with higher frequencies outside of human perception.
Unrelated: WTF? I just had to do a reCAPTCHA on HN to log in!