Shouldn't Unity's dash's "find files" find files that I haven't used yet?
The Files dash of Unity shows up the most recently used files as well as the favourite folders on the home page. This behavior is by design.
If you right-click the Files dash icon in the launcher, then you can select a particular type of file (such as Documents, Images, Presentations, etc) and the Dash will show up all of the recently used files of that type in your home folder.
There is a found lens that do deeper searches.
- Here is the bug report.