Try erasing or deleting your current track.
It's tracking from where you were at to now where you are.
when you turn on your GPS at home to test it, it drops your first track point at your house, ten your turn it off.
when you get to the trail head and turn the GPS back on you now are a track point #2
The GPS now shows a line from home to the trail head
The easiest way to prevent that line is to save the current track.
Then clear the "current Track" so you now start at "Track Point #1" with no point lines
The montanna saves your tracks to your GPS
Under Windows you can se the tracks under "Drive#:GPSchip/garmin/GPX
If starting out down load "Garmin Basecamp" you can see and understand Tracks and waypoints better
In Basecamp you can modify/edit/save al your tracks to your hard drive
Best Mapchip for the Montana so far I think is the Alberta/BC chip from Backcountry Roads
(I do use Garmin MapSource and BaseCamp with ETOPO Maps, OilTracs NT and AB/BC Maps from Backcountry)