>As I was a passenger that day on a private car, I was able to look back and determine the next three blocks were clear of trains, yet the nearest opposing ABS signal remained yellow.

You were a passenger, a casual observer. I'm speaking from an operating employee's point of view. There are certain circumstances under the Rules where visual inspection of blocks is permitted, however, if the signals aren't Clear (i.e. "green"), you can't, or rather aren't permitted to, make any assumptions about what is causing the yellow signal other than a train approaching your location.

As I've posted elsewhere in this thread, there isn't sufficient information available today as to what characteristic of the signal system's circuitry may have kept those signals you saw in the Approach ("yellow") position.