2010 Prius II 102k miles brake puffing, not abs

Just curious, I've had this car for a few years now and this is a new thing. The last 5mph of a moderate/easy stop the car transitions from regen to brake like it should. Around the 3mph mark, it started making a puffing sound behind the brake pedal. At first, I was thinking it was simply switching quickly between the "crawl" mode and stopping. I'm not so sure now. It seems to do it most in the first few miles, but seems to pop up at any time. I'm certain it isn't ABS, because ABS feels entirely different and the pedal goes soft.

My #1 suspicion right now is that the car has a switch that is failing and it keeps thinking I have taken my foot off the brake and is engaging drive "crawl" mode and flipping back to brake mode with the intermittent signal causing the electric pump to puff to adjust brake pressure. It still stops, but it is really odd and somewhat disconcerting.

So far, my searches have only yielded info on regen complaints and that Toyota's frequently puff brakes to adjust pressure.