I was almost offered a job several months ago that involved the construction of an electronic device. It was at a repair shop that had received many requests for "specially prepared suitcases" and other surveillance devices.
The shop manager's ante was not what I expected: "People in Oregon have a fascination with recording each other. This will be more profitable than repairs."
One of his products would be a miniature TV station in a suitcase. He showed me a similar device advertised in a technical magazine. This is a primitive example of what's possible. Some self-styled investigators aren't satisfied with 100% surveillance.
They also transmit messages to their victim.
Doing some investigating of my own, I managed to record some of these messages. It isn't ordinary conversation. Because these sounds are personally directed, they are difficult to record. My recordings were almosted swamped with ambient noise. But modern noise reduction has come a long way. Once you have the "signature" of a human voice, and a digital description of the noise, the signal cab easily be separated from the noise. I used some very basic techniques of noise reduction on one of my recordings as an illustration of what's possible. Listen and decide for yourself what is going on. For background reading, I recommend Steve Wright, who prepared a report for the European Union called "The Technologies of Political Control". The report was published in 2001, and technology has progressed considerably since then.
"...including the emergence of new forms of local, national and international communications interceptions networks and the creation of human recognition and tracking devices"
Warning: these samples may contain obscenity.