Files in /tmp and /var/tmp are deleted by the SO5RMTMPFILES script in /etc/rc2.d when the system comes up to state 2. However, if subdirectories of /tmp or /var/tmp are mount points, they are not cleared by the system. If you set up directories under /tmp as separate filesystems, you can either edit SO5RMTMPFILES so that they are cleaned up, or add another /etc/rc2.d/S* script to do it -- rc2(1M).