Hello,
You can create a "Unmanaged Computers" Notification.
Check the Screenshot:
Also check the Articles below:
About log types
What do the different Notification Conditions for email alerts mean?
Creating notifications in the Symantec Endpoint Protection Manager
You can also create a Sceduled Report can be created on the SEPM, which could save you from such incidents to happen in future.
About the different types of Symantec Endpoint Protection Manager Reports
About Computer Status reports and logs
Also, check the Screenshots with Steps on how to create a Custom Report for Offline Clients.
Step 1: Create Filter
Step 2: Save Filter
Step 3: Create a Schedule Report with the Help of the Filter Created.
Hope that helps!!