This is not possible. You cannot mark report results. What you can do is add an extra column to your report with entries only for machines that have not been seen in the last 30 days.
An example report can be seen below.
Select Top 1000000 tblComputers.Computername, tblComputers.ComputerUnique,
tblComputers.Domain, tblComputers.Lastseen, 'Last Seen Status' = Case
When tblComputers.Lastseen < GetDate() -
30 Then 'Not seen in the last 30 days' End
From tblComputers
Order By tblComputers.ComputerUnique
To use the specified report, do the following:
• Open the Lansweeper configuration console and select Reports & Alerts\Report Builder. Hit the “New” button.
• Copy the SQL code provided and paste it at the bottom of the newly created report, replacing the default SQL code.
• Click somewhere near tblComputers so the new code applies.
• Give the report a “View name” and a “Report name” and hit the “Save” button.
• Double-click on the report in the report list to see its results and export options.