You can have ‘stuff’ in a folder, mount a partition to that folder, and have disk space used for which you cannot account. Rather than randomly umounting partitions to see if there’s anything in the mount point folder, you can bind mount root to another location and check the disk utilization on the new mount.
[root@fedora123 ~]# mount -o bind / /mnt/fakeout/
[root@fedora123 ~]# du -sh `ls /mnt/fakeout | grep -v mnt`
[root@fedora123 ~]# umount fakeout