Sometimes system restore takes up a lot of space, but it will limit itself to a certain percentage of disk space.
It could also be file sharing apps. If you are in queue for a download (even if it is not started to download yet), some file sharing apps reserve disk space before it even starts. If you have several large downloads in queue, that might be your problem.
For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth