Just a thought, but what security package are you using? I used to have Kaspersky. It played havoc with my home network, things were always disappearing from the network map. Since I switched to BT/McAffee Net Protect this hasn't been an issue.
I would not worry about it, the home hubs quite often do not show connected devices. Sometime if the device is transferring data, then it will show up.
Even my home hub 1.0 can sometimes forget to show connected devices on the home network screen, however they show up on the devices menu option.
... but it's a serious problem if the device you want to configure (port forwarding etc.) doesn't appear in the list.
If you've set a device to have a fixed IP address and it doesn't appear, you can't edit it...
Not sure if you did a hub restart or reset?
If you go to advanced settings -> network -> dhcp table does it show up as an unknown device in the list of IP addresses?
I had problems like this with my xbox and a networked hdd recorder.
Suggestions elsewhere were to do a system reset (settings only) and these unknown devices then appeared fine.
It will mean your hub password, wireless ssid etc. will be set back to default settings so make sure you know what they are first.