This!
Thank you, this allowed me to find the culprit!
It turns out I had an external disk failure some weeks ago, and a cron rsync job was writing in /mnt/thatdrive. When the externaldrive died rsync created a folder /mnt/thatdrive. Now that I replaced the drive, /mnt was disregarded by the disk analyser, but the folder was still there and indeed hidden by the mount... It is just a coincidence that it was half the size of /
You only kept the good memories