Unauthorized devices can connect to the network in many ways, such as physical access in a conference room or rogue wireless access points. To enforce policies on every endpoint, you must be able to quickly detect the presence of new devices. Unknown devices are the devices that are unmanaged and that do not run the client AV software. You must determine whether the devices are secure. You can enable any client as an unmanaged detector to detect the unknown devices.
Please refer below document to configure Unmanaged Detector.