"don’t even need anything at the other end of the cable" Congrats, you've discovered that disconnected cables make a great antenna to pick up EMI from the environment. Now grab a long piece of copper wire, hook it up to your scope probe, and observe the same thing.
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New hypothesis: since EEs will ignore it it's a great place to hide transmissions. Test: remove all switching supplies from your environment. Use a clean power source that you trust and reliable high quality equipment.
Be careful with that suggestion, because I bet he doesn't understand that the Pi itself has several built-in switchmode converters (or that e.g. any 5V mobile battery does too). Next thing we know he'll be picking up ripple from that inside a faraday cage and screaming implant.
