I think it is speed dependent, not active until you get over a certain speed.
It is speed differential dependent. You must be going a certain speed relative to the object being detected in front of you by the sensor, and not applying the brakes enough already to be able to slow enough to avoid a collision. The closer you get to an object, the lower the differential speed can be to set it off. If you are going 50mph approaching the rear end of a stopped car, it will go off at a certain distance. If you are going 35mph approaching the same stopped vehicle it won't go off until you are much closer than it will at 50mph.
It is for collision avoidance, to warn you if you might have taken your eyes off the road ahead for a second or two and the vehicle in front of you brakes suddenly. It is not a parking assist feature. If you want that, back into the parking space and the rear parking assist will beep at you when you get close.
If you are going slowly, like when you are parking in a garage, it will never go off as the the vehicle isn't going fast enough relative to the garage wall. If you were to go fast enough, say 40mph, when approaching your garage it should go off when you are part way through your driveway, in time that if you were to brake hard when it went off you would avoid hitting the back wall of the garage. It still wouldn't guarantee exactly how far from the garage wall you would stop.