You mention "offline" folders.
Offline folders are usually used in a domain to sync part of a server drive to a local drive so when you are "offline" aka out of the office thoes files are available, and will sync any changes when you get back.
If you have asked windows to sync a large server folder it will continue to eat up any space you free trying to do so.
From an explorer window select tools then Offline files to view your settings and any offline files.
Alternatively check out Treesize, it is a great tool for Hard drive cleanup