First, your servers should be locked in a secure area so that unauthorized people can't access them even if you do leave them logged in. Some people would frown upon the practice, but I personally prefer to leave my server console logged into root at all times, however I do keep it in a locked room.
Second, "exit" is the command to exit from a shell prompt, however unix works just like DOS in that it is possible to layer multiple shells on top of each other, and you must exit all of the shells individually to get back to the login prompt (possibly the tech person who left it logged in was going back and forth between several shells and had not closed out of all of them).
Third, it is possible with the misuse of some commands to get a SCO shell locked in a mode where it is basically not doing anything but displaying keystrokes. You can get out of these modes sometimes by using the [Delete] key, or [Ctrl]+c, or [Ctrl]+[Break], or ~.[Enter]. If all else fails you may need to terminate the process from another login. When your "exit" command was not working were any of the other commands working, such as "ls"?