Wasn't there. When I hit menu, the only option was to reverse route. I used the nav in car panel, is this not the same as Google nav? Because I just tried Google nav from its icon and the UI looks completely different and when I hit menu, exit navigation option was available.
They are similar but not the same, the Nav from the car panel just uses the Google Maps app and the Google Maps Nav app in it.
When you hit Nav from the car panel it just takes you to a map screen with your start and destination fields where you can select where you'd like to go. Once you select them it gives you written, scrollable, directions.
From there you can push "show on map" which simply shows the route on a normal Google Maps screen (not Google Maps Nav, there is a difference). I believe this is where you were when you said you couldn't find an exit button. From there you can just leave the Google Maps program just as you would any other program, but simply pushing the home button or any other method you choose. Once that map is not on the screen anymore, it will not be using GPS or any extra battery.
Instead of pushing "show on map" you can push "navigate", which will open up the actual Google Maps Nav app (the same one you can open from your app drawer). Once it opens that, it will give you verbal guided directions as you drive (turn by turn).
This is where you can push menu and select "exit navigation", which will do just that and take you back to the home screen.
The reason you need to select "exit navigation" is because just pushing home will take you home, but the GPS will still be running and it will continue to give you turn by turn directions even if the map isn't up.
So bottom line is, Google Maps and Google Maps Nav are two different apps, but both can be accessed from the Car Panel Nav. Both can also be accessed from app drawer or any other way you get to your apps.
Google Maps does not need to be exited (nor are you able to)
Google Maps Nav needs to be exited if you no longer desire navigation
Sorry for the dissertation
Hope if helps