I have two colleagues on a Red Hat Enterprise Linux Server (release 6.4). Both of them can execute the sudo commands. Curiously, only one of the colleagues is in the /etc/sudoers file - let's call him colleague A. There is no extra "sudo" group and both of the users are only in the default groups named after their username. How is it possible that colleague B can execute sudo, although he is not in /etc/sudoers?