You have to press the key combinations (fn+spacebar which is the default or a customized set) or the icon. It'll be cool if they implement "Hey Siri!" as an option.On other note, can Siri listen without pressing Siri icon? Kinda useless tbh if you must press the button first...
I bet when you charge your laptop it has that feature like the iPhone 5s & 5 , but not always on like the new iPhone 6s and 6s plus
There is a way to enable Hey Siri, see this:
Here is my tutorial: How to enable Hey Siri activation on a mac with Sierra
It has a few more details than the 9to5mac article.