[Need4Spd]

I always wondered why I heard the NAV discs running or turn on when I first pop the trunk/boot lid

It's set that way to reduce the boot up lag time when you first get into the car. It gives it a bit of a head start. If they didn't do that, people jumping into the car and wanting to set the nav and take off would get frustrated waiting. As it is, there is a bit of a wait anyway.