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In order to act as an unmanaged detector, SEP clients must have Network Threat Protection (NTP) enabled and be in Computer Mode. User Mode clients or clients without the firewall component (NTP) cannot act as unmanaged detectors
When a client is set as an Unmanaged Detector, it locates unmanaged clients on its own local network segment and reports them to Symantec Endpoint Protection Manager. An Unmanaged Detector cannot detect unmanaged clients on network segments other than its own.
Make sure unmanaged detectors are configured per subnet.
Refer this article: SEP 12.1 - What does it mean to set a client as an Unmanaged Detector?
http://www.symantec.com/docs/TECH183746