How can I find orphaned files in the Vault?

Hi, I want to track the amount of Files that are not related to a Document or Change Processes anymore. I noticed that these unrelated files are somehow assigned to the SystemFileContainer Orphaned Files Container. SystemFielContainer is used as PolySource in FileContainerItems. But I have some trouble to find the right query, to detect all of my orphaned Files. Does somebody have an example query for finding orphaned files? Or maybe just some hints, how these different ItemTypes stick together? Thanks and best regards! Angela