You can select which lines to display and which to hide in the main area by checking the relevant options in the lower half of the View menu.
Checking View > Show Viewed Lines causes all lines that are marked as viewed to be displayed.
Checking View > Show Error Lines makes the Grep Browser display lines containing error message lines, for example, lines stating that no matches were found in a file.
Checking View > Show Hidden Lines ensures that even lines hidden using the Matches menu are shown.