I received my new 6S yesterday and had a nightmare upgrading - took me over 5 hour when normally it is smooth and quick. Anyway... With the new 6s / 6s+ Siri can be listening all the time. You train it to recognise your voice so it won't trigger for anyone else. This works pretty well, in isolation. The problem occurs when you try to use Hey Siri on the watch and the phone is also in ear-shot. In that instance both items start listening and try to respond. I guess the work-around is not to use the watch when the phone is near by, but I would have expected Apple to handle this use-case more smoothly. I'll look for any settings to refine the behaviour.