wjpbill, again thanks for that link. I've reviewed the footage from the picture in Scarbs' tweet. It came from roughly 10 minutes into FP2 during the Saudi GP last year. I'm actually more convinced after seeing it that the glow is produced from skidblock incandescence or if possible (and I have no idea of the physics behind my next theory so I could be way off), some sort of interaction between a hot skid block and the optical sensor that makes the sensors light more visible when the area immediately surrounding it becomes extremely hot.
The reason I'm more convinced after watching the 2021 Saudi FP2 footage, is because the car is shown grounding and sparking heavily around the gentle, flatout left curve leading towards the final corner T27. The sparks are seen to emanate from the righthand side of the bib, which makes sense as that part of the circuit is a lefthand curve, which causes the car to roll to the right, meaning the right side of the car and bib is closer to the surface and or creates more friction on the right side of the skid block. Into the braking area for T27, after the sparking has stopped and the car is steered into the corner, the same glow we've been discussing is visible, yet unlike Australia where it emanated from the centre of the bib (That car was travelling in a straight line), in Saudi it was offset to the right at the same spot the sparks were being generated. Here are a sequence of stills from the footage:
NOTE: The the car had been sparking and grounding for a while before the picture shown in the first still.
In this first image, that very small bright spot under the bib, that's offset to the right (our left), is the car sparking.
Unfortunately almost the entire lap is shown onboard with the exception of the run down to T27 I described, so there's no way to know how the bib looked at other points during the lap. I'm also aware that some may argue that the sensor was installed offset to right, which may be valid.