This is the question what [Unfiles] smartfolder is for.
This folder seems to show files that remain in [Files] folder and NOT in sub-folders beneath it. It may be useful for pre-filtering to reduce number of files in the Search action.
In my case, however, almost files (1049/1458) remain in [Files] folder and sub-folders are used only for specifying series of web pages to some theme (409/1458).
If user has custom to move captured files to sub-folders and [Unfiled] folder has small number of files, this folder may show Recent captured files.
However, I use [Recently Added] folder to find just captured files and sometimes use [date] option of Record Lists.
So, I have confused about what [Unfiles] folder is planned for originally.
You can use it for whatever you want. What I had in mind was that some users will want to categorize everything into subfolders, and Unfiled will show all the files that have not yet been categorized. There are many situations in which this would not be the same as what Recently Added shows.