[.thumbnails file]

Samsung tablet Android 5.1.1.  .thumbnails folder shows as .98gb so I run ccleaner (or Norton clean). Both show as DELETED yet, not much increase in memory. Reboot new .98gb file.  I read that creating a .thumbnails FOLDER would prevent that - nope.

So why not a .98gb gain in memory when deleted?

[how to clean hidden cache on Android]

ANALYZE disk

It will show

HIDDEN CACHE as one of the items.

Click " FINISH cleaning"

It will clean everything you checked.

To BE SURE it happened, just close app, reopen and you'll see that it's all gone.