At Cold War time, in USSR, American spy device was discovered that was placed on the street outside the wall of main frame computer installation nearby drum printers (https://en.wikipedia.org/wiki/Printer_(computing)#/media/Fil...). The device was recording signals from them and retransmitted recording in compressed form to satellite.
The device was masqueraded as a canned "Килька черноморская в томате" (Canned fish: Black Sea sprat in tomato) and was transmitting short narrow beams upstream on schedule.
I suppose it could be capturing EM emissions from the printers rather than acoustic, but that would make it fundamentally more like the other TEMPEST hacks as a way to exploit unintentional emissions, rather than as a sneaky covert channel where you have control over the computer equipment, but don't have access to external interfaces to offload your stolen data.
Unless I'm misunderstanding some part of your description, anyway.
https://arstechnica.com/information-technology/2015/10/how-s...