Got someone to shoot the simulator at the ALP for me, while I sat in the car. The answer is:
1. Solid LED (red): Being actively shot from front
2. Solid LED (yellow): Being actively shot from rear
3. No LED, but steady tone for 2-3 seconds: Was shot, but no longer being targeted

Ugh. Don't like this behavior. If you pick up scatter, or the cop shoots you and moves on to the next car (with full intentions of going back to you), you'd have no clue where the threat came from, if you weren't staring at the LEDs the entire time. This is important, especially at night, because you don't want to slam the brakes if it's a rear threat (or you want to kill the brake lights).