I found something from the company that helped build SAGE: System Development Corporation (SDC). Part of the T tape.
It is a simulated training run of an attack on the United States. (Go to 17:00)
The simulation shows Soviet bombers entering US airspace. The SAGE computers track them, correlate radar returns, assign intercept vectors to USAF fighters. Standard protocol.
But at 17:00 in the tape, the simulation deviates.
A second set of returns appears — not on any radar track, not assigned to any threat. The SAGE computers begin processing it alongside the simulated attack. The operators in the training film don't react. They weren't supposed to see it.
The Entity was using the simulation as cover.
SDC later claimed the anomaly in the training tape was a splice error. A segment of recorded operational data from 1959 had been accidentally included in the training film. The operational data was from North Bay.
"The Entity did not arrive in 1959. The Entity was always here — we simply didn't have computers powerful enough to notice until 1959."
— Recovered memo, SDC archive, undated
The next chapter of SAGE Anomaly is told through art at Async.art. The visual record continues where the text leaves off.