stock android - spy / invisible file occupying 4gb+ space in internal storage

I am running Android 6 Marshmallow on Sony Xperia z3. I use my memory card second partition to link most of my apps and increase my space. (app2 SD / link2sd app) some of my folders I link manually using a folder mount program. from internal memory to memory card first or main partition. for purpose of my testing I unmounted or delink all tgese linked folders.
so my internal memory shows as just 1.2 gig free from some 12 gig internal memory. matter of fact currently space occupied shows only has 4.5 gig . using any File Explorer. I am using this file explorer in root mode which means I am able to see all invisible or hidden folders..
in spite of that I am not able to account for the missing 4 + GB or space what is occupying this... there is some process That seems to be creating a totally invisible file which even I cannot see as root..
how to identify this kind of files and go ahead and delete them. if only 1.2 GB is free it means around 10 GB is occupied but in the attached picture only 4.8 GB are occupied. how do you account for the discrepancy ?