If you look at Settings-System-Storage, you will see three main sections with others available.  Go through those and see if any extra files show up. 

You can click on each section and it will show what files/folders are present.

Some folks also have problems with restore points being accumulated.  You can check that by using the VSSadmin utility in an admin command prompt or perhaps modifying the amount of space the restore points can use. 

Try a Disk Cleanup with "Clean up system files" selected. I agree with @Saltgrass that it's something like restore points or a previous Windows version hanging around, but Disk Cleanup should find it.