Hello,
There is a feature of "Unmanaged Detector", check these Articles:
What does it mean to set a client as an Unmanaged Detector?
http://www.symantec.com/docs/TECH105722
Best Practices: When to use the "Find Unmanaged Computers" or "Unmanaged Detector" features in Symantec Endpoint Protection 11.0
http://www.symantec.com/docs/TECH104340
Find Unmanaged Clients on a remote network location using the Unmanaged Detector
http://www.symantec.com/docs/TECH96234
Setting notifications when using the "Unmanaged Detector" feature in the SEPM
http://www.symantec.com/docs/TECH104897
Hope that would help you !!!