Hi Phil,
When you click on Network in File Explorer it goes out and does a Network Discovery scan of the local subnet and reports and machines occupying an IP address within the same subnet.
I can't explain why you are not seeing them in your router though, unless the Network Discovery results aren't accurate. They couldn't get an IP address unless they were able to authenticate at some point.
Try to ping one of the IP's from a command prompt:
- Press [Win key] + [X] then select Command Prompt.
- Type ping xxx where xxx is the IP address.
- Hit [enter].
Do you get request timed out or replies?
If you get request timed out then that device is no longer connected to the network. If you get replies then it is still connected to your network.
Let me know if this helps.