In opening an older model in 2.81, a couple of the bones show a red curve in pose mode:
As you can see, it happens to conform with the positioning of this bone's tip when rotating 180 degrees to the left and upward, in the axis perpendicular to this view.
The bone previously had an IK on it; the red line remains even after removing such. Other than that, comparing all properties between it and other bones, I see no other explainable differences.
Curiosity has me. What property displays this indication curve in pose mode?