I was using the Navigator app and I was still hearing voice directions even though Task Manager showed no apps running. How can Task Manager show no apps and there could be an app still running?
Not sure which task manager you are using -- but some devices/Android versions distinguish running apps from services. On my Droid2 running Gingerbread, I find running apps at Apps->Apps in the "Running" Tab, while services show up at Apps->Active Services. Parts of Google Maps (which is the navigation app you probably refer to) are implemented as services, as e.g. the "location service" which keeps checking your GPS or Wifi based location in the background. I never used navigation itself, but I imagine this is handled similarly.