Someone

on *:TEXT:*brb*:#:{
  if (%gone. [ $+ [ $nick ] ] == $null) {
    /set %gone. [ $+ [ $nick ] ] $ctime
    /msg $chan $nick has stepped away from the Key Board.
  }
}

on *:TEXT:*back*:#:{
  if (%gone. [ $+ [ $nick ] ] $ctime != $null) {
    /msg $chan $nick has returned. $nick has been gone for $duration($calc($ctime - %gone.[ $+ [ $nick ] ])))
  }
}