Systemdisk/OS disk almost full

No, I did not get any explanation - and I am just as curious. My hypothesis is that my Docker container was given a wrong path, hence it made a path on / instead on the mount point in my MergerFS pool - this filled up my drive. When I deleted files/folders and removed the containers/Docker images as well as uninstalled Docker using the plugin - something was still occupied by a service and the data allocated to the disk was still occupied ...
LSOF did not help me, so if there was a service keeping this "shadow" folder active I could not find it. Weird.