Hmmm... this sounds made up. AC current is converted to DC current to run a computer. If there was a signal present on the AC current, it would be destroyed in the AC-to-DC conversion. And second... computers don't respond to a signal unless there is a hardware device that can interpret and process the signal. No device exists in your standard computer, and it certainly doesn't exist in your AC-to-DC converter.
That's where you're wrong, Sir. I'm a Transmission Engineer and have taught engineering design of the outside plant to Outside Plant Engineers and Telephone Technicians how to use their meter to identify and locate troubles since July 27, 1997. I've been in the industry since 1984.
Even ADSL modems use both Longitudinal and Metallic current in it's transmission of data. Why the hell do you think you can still have internet without dial tone caused by a hard short on your twisted pair going to your house? A "HARD" short creates TWO Metallic paths. However, You can still have excellent internet with your ADSL modem with that hard short on the line. You're subject to have no voice transmission but will still have excellent internet transmission with a HARD short. WHY? Because LONGITUDINAL CURRENT isn't studying a short. Longitudinal Current flows on each conductor past that short to ground. As long as there is capacitance to ground; longitudinal current can flow on that conductor until the capacitance to ground ends.
I'm NOT going to give a detailed course on here explaining how the conversion from longitudinal currents on the power line can be used to see DATA transmitted in a modem LONGITUDINALLY. POTS (Plain Old Telephone Service) used to be strictly Metallic Current. Now, in the age of VOIP (Voice of Internet Provider), the voice is also transmitted LONGITUDINALLY; NOT METALLICALLY through CLEC's (Competitive Local Exchange Carriers) but metallically through most ILEC's.
But again, I'm not going to explain this to you nor do I have to. If you choose to be naive, that's your choice. Not mine... Most of the surveillance done on copper wire is done with Longitudinal Current. SOME is done with Metallic Current over plain old telephone service but not as much as it used to be.