True, rider frequencies are used all the time in industrial power lines. Encoders and decoders find frequency fluctuations or amplitude of the rider frequencies to turn on/off lights and some devices. Look up X10 technologies for a crude explanation of how this works. At one time, SFO used this technology to control devices on the power grid to reduce electric usage at off peak hours. Worked pretty well except when hooked up to frequency sensitive and power line frequency detection devices in Avionics automatic test equipment and line test equipment hooked up to house power.
The key words are encoder and decoder. Encoders are matched up with decoders which makes it all work.
IMHO,
For someone to capture computer data that is hooked up to a grid (and not the internet), they would have to place an individual power line monitor for the computer and filter out power surges/spikes to decode in/out data.
I would doubt that it can be used to extract data from the computer except for input/output power fluctuations.
JMHO,
B) xUT