I had a discussion with a friend about The Thing[1], an invention by Léon Theremin and how ingenious it was that it required no external power source and can be turned on/off at will by transmitting at a certain frequency. The trouble with this is that it's still possible to identify the retransmission. In fact, this was how it was discovered.

What if the room had a length of surgical or other type of long tubing embedded into the wall so that the detector was a fair distance away? Essentially, a very crude stethoscope originating in a wall outlet or other place in the room that already has an orifice(s) and terminating at another location with a passive resonator.

By decoupling the transmitter from the audio source by using a non-electronic medium, it's possible to still eavesdrop on keystrokes even in an em-shielded room.

[1] https://en.wikipedia.org/wiki/Thing_(listening_device)