I just found out there is a video of this and being the video guy that I am I checked it out and here's what I found:
Here's the frame right before the red light comes on:
Here's the next frame when the red light starts to come on:
Here's the next frame when the red light comes on fully:
And here's the next frame after the one above:
It's hard to tell by just looking at the photos, but in the last frame you can see the reflection of the flash off of the left front corner of the car.
You need to download each picture and scroll through them and it's easy to see. I just use Windows Picture and Fax Viewer and use the right arrow to go to the next picture.
If I were a betting man, which I am, I would bet the camera that took the picture has the capability to trigger external flashes. And to trigger the external flashes the camera uses infrared.
I don't know how external flashes work, but it makes sense that you would have to send out a signal from the camera to the external flashes before the actual flash happens so the flashes are in sync.
The video has a frame rate of 30 frames per second. Which means each frame is .033 seconds apart.
So I believe my hypothesis was correct in my post yesterday.