Refurbished Mac - "Other" Storage

I bought a refurbished MacBook Air, which has 13GB of "other" storage.

There are no files on this computer that, I have verified/repaired disk permissions, deleted caches, iTunes backups, and emptied the trash. I spent hours on the forums here doing everything suggested, and still cannot locate what this "other" storage might be.

Any suggestions would be much appreciated. Pictures provided for both issues.

Also, I am unable to erase free storage space on the hard drive - it is grayed out.