Phantom devices showing up

Another method is to turn off one WiFi device.  Wait 5 minutes, and then refresh the Attached Devices page.  If one of these strange devices has disappeared, then it is whatever got turned off.

Additional info.  There seems to be a third ghost.  An item in SoftPerfect Network scanner says there is a "Streaming dongle - Google - Chromecast", but I don't own any sticks or dongles from Google.

Re mobile devices: mixed results.  The List shows one laptop, and shows an "Android" device (with a 16 digit alphanum item after "android".  However there are three mobile devices actively on this network, not two as just noted in the List.  (I have two Samsung phones and a Lenovo laptop that are currently attached to the network.)    Possible coincidence: the Android item listed in the Netgear List is 192.168.1.3, and when I run SoftPerfect Network Scanner is says that that IP is for the (ghost) Netgear Meural photo frame -- which I don't own.  Is the Meural type of device an Android device?  I don't know.  But if it is, then neither of the two Samsung android phones currently on the network are showing up in the List.  

Netgear routers include a feature which attempts to determine the manufacturer of network adapters by their Organizational Unique Identifier (OUI) code.

There are numerous tools on the internet which perform this function, such as https://www.wireshark.org/tools/oui-lookup.html

When WiFi devices randomly change their MAC address rather than using the actual hardware MAC address, this feature may wrongly identify a device.

One way to see if this is happening is to go through the list of Attached Devices and see if there are any mobile devices (phones, tablets, laptops) which are connected to the network but do not appear on the list of Attached Devices.