First, I am not an expert, just another truck owner, but I found that the indicator did light up when the truck thought it needed to apply the trailer brakes, based on how hard I was stopping. For example, with an empty trailer, and an easy stop, the truck didn't apply any trailer braking, but...
Mine does the same thing. I think it is normal because it works when I'm actually stopping. I believe the accelerometer is used to determine how much braking is needed. This is overridden when using the manual control.