Consider that the CIA's internal policy is to treat every computer you touch as if it's been compromised. Even the ones at your desk in the CIA.
Remember that airgapped, inside a Faraday cage computer deep in the vault in Mission Impossible where that analyst was typing a report? They really have those buried away for the Uber Top Secret, don't give this to Russia/China stuff.
So, if even the spy agencies don't trust anything here at home, you think they might know something we don't. Trust nothing, it's all already compromised, seriously.
I think it's more about complexity than actual case-history: modern machines are so complex in both hardware and software, they're basically un-auditable.
You have programs executing code compiled by other programs on top of programs booted by special programs embedded in hardware chips built with circuits as small as atoms. You can barely detect unauthorised operations at the very top layers (and at huge cost in terms of effort), everything underneath the OS is basically a black box for all intents and purposes... and you can't trust a black box.
Even those are compromised. Key tap patterns can be recorded and calculated. You need to have a zero electronics, faraday cage room in order to start to trust things.
This is why the NSA does their own chip fabrication. Although probably only used for crypto gear not server chips. I wonder what they do for the servers in the Utah DC.
Remember that airgapped, inside a Faraday cage computer deep in the vault in Mission Impossible where that analyst was typing a report? They really have those buried away for the Uber Top Secret, don't give this to Russia/China stuff.
So, if even the spy agencies don't trust anything here at home, you think they might know something we don't. Trust nothing, it's all already compromised, seriously.