Question: what's the fastest way to find out who, in the room, is carrying an iPhone 6s?
Answer: Yell 'Hey Siri' :-)
A more serious question: does Siri learn to be speaker dependent over time? Otherwise I can just see everyone's Siri waking up whenever I try to ask mine a question :-)
[Has Apple already taken some of that into account? I have asked Siri to remember stuff for me when I was half asleep and my iPhone was charging on the nightstand - my wife's iPhone never woke up.]