Beacons and probe broadcasts are actually somewhat different.

All APs must send out beacons, otherwise their clients don't know how to do collision avoidance. However an AP in closed mode may send out beacons with the SSID modified slightly - the Lucent ones replace the real one with a single space, the Cisco ones replace it with as many nulls as there were characters in the original. Most other manufacturers seem to still send out the original SSID.

The thing that stops NS from seeing them is that they don't respond to broadcast probes.

I'm working on getting the beacon packets so that NS can display what it sees there too - if I can get it to work then we can stop sending out probes, and NS can run passively.