For some reason, the Active Listening feature while the phone is off and locked works on my Nexus 7 with KitKat and the Google Now Launcher when the tablet is plugged in to power. It does NOT work when it is not plugged in. I have discovered this by accident when querying my Moto X in the same room where I have the Nexus 7 plugged in and confused by why BOTH devices respond.
How I expect this to work is Active Listening is only enabled on this device when it is on and at the home screen.
It make a little sense why it is behaving this way, because the Moto X has dedicated hardware for Active Listening that is designed to minimize the impact on battery life. Presumably, if Active Listening were enabled on other devices while powered off, battery life would suffer. Obviously, if the device is plugged in then there is no impact on battery life.
The question is, how practical is this feature? How often do people use their Nexus 7 while it is plugged in?