Notepad++ Changebar
Solution 1:
The LocationNav plugin lets the user mark changed lines in different ways, e.g. highlight, underline and symbol (rectangle, roundrect, circle, triangle and arrow). (In fact, I find this feature more useful than its original purpose of returning to lines the cursor previously moved onto!)
Solution 2:
You can use this plugin (Change Markers).
This is a pretty good list of Notepad++ plugins
Solution 3:
You may have something like Change Markers, a Notepad++ plugin, in mind. It's listed on the Notepad++ plugins page.