I, too, wonder if the Clarity PHEV illuminates its brake lights when the ACC slows the car. I don't believe that the paddles ever cause the brake lights to come on, but they cannot slow the car as much as the ACC must do to maintain a safe forward distance. The cheap, ugly way to figure this out is to tape some aluminum foil over the high-mounted center brake light in such a way that you can detect its glow in your rear-view mirror. I've never tried out this ridiculous idea.
Because I couldn't stand not knowing when my small, sporty BEV turned on its brake lights in response to regen braking, I
mounted an interior LED in the housing at the top of the hatch and wired it to the high-mounted center brake light. When looking in my rear-view mirror, I was disappointed to discover the brake lights come on in response to the slightest bit of deceleration. Since mounting that LED I've adjusted my driving style to minimize brake-light illumination. I no longer rocket away from a stoplight and then slow down to the speed limit. Instead, I now rocket away from a stop light and stop accelerating when I reach the speed limit. Interestingly, someone on another Inside EVs forum looked at my BEV's energy usage profiles and claims that rocketing to 35 mph uses no more power than slowly accelerating to that speed. Of course, I took that claim as gospel and looked no deeper into it.