Yep, guessing it was just training. Which is good to know! It was certainly descending at a speed I've never seen a helicopter come down at before. I was cursing myself for not just stopping the car to see it recover from the high-speed descent (I had a pretty clear view almost to the ground, but then buildings obscured it after I saw the first five seconds or so of the "fall").
Anyway, nothing in the news, so must have been training. Wow, autorotation training must be really exciting!