The Unity lens does not search all your files and folders, but rather only has access to files that you have opened at least once in the past (this is because it uses Zeitgeist for search).
There is a different lens you can install called "Unity Place Filesearch" which will search for every single file on your system. This provides much more thorough search results for files that you have never opened before. Once this new better search lens is installed, you can uninstall the old one if you do not want it anymore.
To install Unity Place Filesearch:
sudo add-apt-repository ppa:pydave/unity-lenses
sudo apt-get update
sudo apt-get install unity-place-filesearch
Then enter the following to make the lens appear:
OPTIONAL You can also set which folders to ignore (if you have private files) by editing the following file:
If you do edit this file, you can apply your changes by saving it then entering the following commands to stop and restart the lens.
To remove the old lens:
Backup the lens files first (just in case):
cp /usr/share/unity/places/files.place ~/backups/
Remove the lens:
sudo sed -i '14i ShowEntry=false' /usr/share/unity/places/files.place
And finally apply your changes by running:
All finished! Enjoy your new better search!
Sources all by Webupd8: