Hi from a newbie in this great forum! May I use this old topic for similar question...
I used OMV with Transmission in Docker / Portainer without any issues. After a memory upgrade / reboot I downloaded some .iso files (total 20GB) and didnt notice that they do not appear in the proper download location... 
The result is that I lost 20GB of space on my OS disk and the OS became big enough to not be able to backup. It was 2-3Gigs before that.
It was 3 weeks ago, so I cant recall exactly if I did something else which can cause the issue.
As proposed above in /srv there is only one "dev-disk-by-id..." but I think that this is my Media HDD, should not delete this, right? 
I have tried with "du - h -d1" in OMV shell and in Portainer shell to find the culprit for the lost Gigs, with no success. They're just gone.
If it is important - I am running OMV in VE in Proxmox VE, the Media HDD is mounted in OMV only, and I have some unclear memories that I needed to re-mount after the reboot in question.
Please for your help! And Thanks!