It should come on as you when you are pretty much coming to a complete stop. It most likely won't when the car is decelerating for a brief period
If the car applies the brakes to slow down, they will come on. For example, someone pulls in front of you and slows suddenly, or you crest over a hill and the car picks up speed on the downside, or you're coming to a stop. Confirmed one evening in the dark with a dirty rear window (could see the haze from the 3rd brake light when brakes were applied).
If the car is letting off the gas to coast, they won't.
Brake light operation is exact same as though you were driving it and coasting or applying the brakes.