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Sounds related to motion camouflage, which dragonflies use to hunt. Allows them to appear stationary, with the only sign they are on an intercept course is looming as their image slowly grows larger.

https://en.m.wikipedia.org/wiki/Motion_camouflage

The way I heard this is, at a military site, "No change in azimuth. No change in elevation. Let's get the hell out of here!"

CBDR only requires constant velocity. Similar triangles are all you need. On short time scales even orbits are approximately straight. It still applies and may well be the reason this one was missed until so close.