/etc/mtab represent currently mounted filesystems
/etc/fstab represent those configured to be mounted by the installation or sysadmin.
Now, the block device you're talking about is a fuse filesystem? when the system answer that the device is already mounted is because it is there mounted...now, you cannot umount a partition or device if the following is happening:
* /dev/sdb1 is mounted in /dev/media/me (for example) and you are in a shell inside /dev/media/me and try to umount /dev/media/me or /dev/sdb1 the system will answer you that the device is busy, and wouldn't allow you to umount it. The same thing will happen if you or someone else on your network have some file opened and the file belongs to /dev/sdb1 that is mounted on /dev/media/me, if you or that someone that have the file opened through the network don't close the file, you cannot umount /dev/sdb1
The partition to be able to be umounted need to not be busy, and busy means having some file that belongs to that partition opened, or having your user inside that partition.