I would have thought one single unexpected packet in these high security environments would raise significant alarm bells and any anomaly would be found very quickly.
This sort of monitoring doesn't happen in the real world.
Except on extremely controlled networks, this would be very hard to detect. It gets even worse when you consider that the Chinese had/have a distributed network of compromised machines. Imagine using a Google edge server as a dead drop...
Now this does not make it impossible, just very complex. In a more "controlled" environment such as a naval ship, i could see this actually working better, especially if the system is supposed to talk to very few external systems.
Security engineering is about tolerable failure modes. - Dan Geer (2014)
If you’re running an IDS on a big 100 Gb datacenter network, you’re literally processing millions of events. Very few places would notice such a thing unless they were investigating something related, and the ones with the capability are going to be for static workloads as getting anything done will be slow and painful.