You can find two types of built-in reports in the Reports section of the web console:
Device: Not seen in the last 30 days (or 60 or 90 days)
Computer: Not seen in the last 30 days (or 60 or 90 days)
In order to list the result of these reports on your Dashboard, add a
Data Report widget to your Dashboard and use one of these reports.
In case you would like to modify the reports (i.e. in order to use lower number of days), open the reports, hit Edit Report and change the filter on
tblAssets.Lastseen.