When we use tl-umount-localdrives for all users (-a), we use the system's "umount" rather than our "tl-umount-personal". Unfortunately we don't give it the proper -n flag:
tluser@dhcp-252-255:/var/opt/thinlinc/sessions/tluser$ tl-umount-localdrives -a -v
umount: /var/opt/thinlinc/sessions/tluser/1.ended/drives/cdrom is not mounted (according to mtab)
r29140. Note that I didn't get any errors on CentOS 6 even without -n.
Verified and works as expected.