no one is able to catch the signal. Of course, it is not very convenient to do it every time but you can decide whether to do it or not. There is one more benefit - in this way you will safe your battery life for long time.
I don't know if the NX can display the mpg every minute. If so, look at this on a short journey. When I look at mine it takes some miles before my average gets above 30mpg even when the roads have cleared.