It sounds like it's a claim along the lines that you can't tell "I love Lucy" is on because you are listening to the audio and not looking at the screen.
But the pattern of activity of thousands of voxels across cortex does contain reliable information! And a decent amount of it too, at least in sensory cortices.
Constant lower draw devices- chargers, lights, speakers and such- are going to be harder to distinguish, though.
https://ubicomplab.cs.washington.edu/pdfs/electrisense.pdf
https://www.researchgate.net/publication/266461208_Multimedi...