I had mine do that once when I was driving in traffic quite slowly along an urban main street. I realised the car had detected a man stepping out into the road between two parked cars. In fact he was only going round to the driver's side of his own car to get in, and I had realised that - there was enough room and he wasn't doing anything dangerous. But I forgave the car, because it could have been someone about to run across the road.
I have noticed pedestrians doing similar things since then and the car didn't react, so I'm not quite sure what it was about that occasion that made it react so strongly.
While on holiday in a hired ICE car, much the same thing happened except it was only an audible warning, not a full-on emergency brake. My friend who was driving at the time, and whose own car is ten years old without any of these modern aids, was very startled by the warning and said, "what's that?" but I pointed out the pedestrian and explained the car had seen the woman and was warning her of a possible hazard. She was well impressed.