I've gotten in the habit of using my blinker for most, if not all turns when on the freeway here in Houston.
Once the blinker is on and I have a car in the blind spot, my wheel vibrates and the mirror indicator flashes. I believe this takes place if the indicator light was lit or not.
I too am still trying to figure the indication out, but it is my opinion the indicator lights up only when a vehicle approaches you, but certainly not as you pass a vehicle.
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