A question for those with more knowledge than me about Synology NAS (not hard, given my limited use).
I set up a 12TB dual raid Synology DS220J as a back up solution a year or so ago (currently running DSM 7.1.1). 
I figured that would be enough space given that I'm only backing up about 6TB of data from several computers, which also allows for a bit of versioning.

However I've been getting drive warnings that the system is almost full. I tried discarding versioning, although as I previously was only keeping a second version for 60 days max, I didn't think that would affect things much, and I was right the space used barely changed.

I'm just baffled as to what is using this space: Storage Manager shows 9.2TB/10.8TB used, thus the warnings,
but the Synology Drive Admin Console agrees with my estimate by showing that the file versions are less than 5TB, and I installed Storage Analyzer, which agrees with the Drive Admin Console by showing <5TB used.

So how do I find out what is using the rest of the space?