Still can't replicate, but can add some discovery:
If sudo has been used in the session before and a password is not needed, system-monitor correctly identifies updatedb as the process-name. If a password is needed, sudo is shown as the name. The PID displayed by top, ps and system-monitor are the same, so they still point to the same process, there is no hidden one. Also, the reported CPU-Usage is the same for the sudo process in system-monitor and updatedb in top.
Still, the name confusion issue should be reported upstream, can you file a report on bugzilla.gnome.org?
Still can't replicate, but can add some discovery:
If sudo has been used in the session before and a password is not needed, system-monitor correctly identifies updatedb as the process-name. If a password is needed, sudo is shown as the name. The PID displayed by top, ps and system-monitor are the same, so they still point to the same process, there is no hidden one. Also, the reported CPU-Usage is the same for the sudo process in system-monitor and updatedb in top.
Still, the name confusion issue should be reported upstream, can you file a report on bugzilla.gnome.org?