The G2 is not supposed to come on even if the power button is pressed if the device is in a pocket, backpack, etc.
There is a proximity/light sensor that, if covered, prevents the G2 screen from coming on. You can test this by covering the proximity/light sensor with your thumb and trying to use knock on, the power button, or volume up or volume down keys. The G2 does not turn on (exception: if you use the power key, it does turn on very briefly but immediately turns off). As a side note, some people that might have knock on problems might have their hand too close to the proximity/light sensor when knocking.
That said, there are some posts out there where users report that the little test I described does not work (device still comes on with the proximity/light sensor covered), but those are the exceptions.