I just read the release notes for 0.4.0, and saw that if a machine knows the SSID of a closed network, then Netstumbler can see it (though with a blank SSID field). Going by those notes, I think this is probably due to the fact that this is the same machine I set the network up with.
Just to verify, if someone else locally were using Netstumbler, and I close the network and change the SSID - they will NOT be able to see it with version 0.4.0 of Netstumbler? Could someone please confirm this?
Ta folks.