Hi,

When a device starts up, its operating system sends ARP traffic to the network to let other computers know of the device's presence. A client that is enabled as an unmanaged detector collects and sends the ARP packet information to the SEPM. This management server searches the ARP packet for the device's MAC and IP address. The server compares these addresses to the list of existing MAC and IP addresses in the server's database. If the server cannot find an address match, the server records the device as new. You can then decide whether the device is secure. Because the client only transmits information, it does not use additional resources.

You can configure the unmanaged detector to ignore certain devices, such as a printer. You can also set up email notifications to notify you when the unmanaged detector detects an unknown device.

NOTE: 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.

Reference article: SEP 12.1 - What does it mean to set a client as an Unmanaged Detector?

http://www.symantec.com/docs/TECH183746