How to view items that are both unfiled AND untagged?
I consider such items to be orphaned by accident. I will probably never find them again and might as well delete them, but I may just have forgotten to sort the item into my reading list or to tag it after reading.
If you select the tags tab for the top item under Unfiled, you can then scroll down and see which do/don't have tags.
Untagged items in general can found with a saved search: Tag/does not contain/(leave empty).
But I don't think there is equivalent syntax for Collections in a saved search that would find unfiled items that way (eg Collection/is not/(leave empty) does not work in the same way). And Unfiled does not qualify as a collection for that list (so Collection/is not/Unfiled is not available).
You could however drag all items under Unfiled into a temporary collection and run the above 'untagged' search just on them.