Are you the only user at > System Settings > Users & Groups?
Also any file that does not fit in one of the predefined categories will go into the Other Users storage. This includes your Desktop Folder and Downloads Folder.
Another spot is in you Cache folder. In the Finder use Shift-Command-L to open your Library folder. Right-click the Cache folder and choose Get Info to see how much storage it is taking up. I wouldn't delete everything in there, but you can sort the list by size and see what is taking up the most amount of storage space. The only ones I would delete are apps that I no longer use.
And another is your Application Support folder located in the same Library Folder. Most of these files are needed and should not be deleted. You can look for apps that you do not use anymore and delete those. The Application Support folder may also hold backups for devices you do not have anymore if they were backed up with iTunes. They would be found by opening MobileSync > Backup.
As a warning, make sure you always have a recent backup before making any changes to System Files.