On my 2015, when I have the push button on (pressed once for ACC mode "power is on" displayed I get a light buzzing, or humming sound emanating from under the dash somewhere, which sometimes takes a few minutes to start hearing it. It sounds exactly like what a transformer sounds like, humming, but not too loud. A first I thought the fan was on at low speed, and it sounds about that loud, but I turned everything was off, including the audio to make sure it wasn't that. As to why I was in accessory, it's because I spend my lunch hour in my car and run the radio but it makes the noise with everything off (except ACC mode of course).

As a side note, if I extend my lunch hour to a bit over an hour, the entire car's electrical shuts down. When that happens, and I go to drive the car, the touch display screen is unresponsive and the auto headlights delay to come on until really dark. Thankfully though once I cycle the push button all is back to normal. I now make sure to not stay in ACC mode for m ore than an hour. I believe the power kill after an hour is normal, though it's not spelled out in the manual.