There are two events that you'd need to add. The first would be on PART, which has the same syntax as on JOIN. The second is on QUIT, for which the syntax is slightly different.

Code:
on *:QUIT:{
  if ( $nick ison #Santharia ) {
    describe #Santharia watches as someone leaves the channel.  4"I'll miss you $nick $+ ."
  }
}

One thing to note about on QUIT is that you can't use # or $chan to reference the channel name, as people don't quit channels, they quit the IRC server.