While stuffing around with the ride height last night I found out that
there is an indication of plank wear while actually 'in***pit'. When
the plank comes into contact with the ground, the car damage
indicator(?) lights up half of the vertical amber line that is visible
when you are jacked up on the stand at the pits. If the front end of
the car is touching the ground then the top half of the line lights
up, and presumably the bottom lights up for the rear of the car in
contact (haven't tried yet).
Checked plank wear, wheel ride heights and plank wear indicator, they
all told me that the plank had been rubbing on the ground at the
points in the race where the light had lit up.