Symantec Management Platform (Notification Server)

It seems to me there was a collection under the Resources tab that would return every computer the NS was aware of, whether managed or otherwise, but I am unable to find it now.

I am using version 6.0.6074 (SP 3 + R7)

This should help explain what you are looking for, although depending on what resource discovery method you are using, it could differ slightly. https://kb.altiris.com/article.asp?article=19259&p=1

That being said, if a system is unmanaged, you'd have to include other sources of data, such as a collection where Agents aren't installed, or with a filter of ismanaged=0

AFAIK, there is no single, reliable source of truth.