The TMI-2 mudlib has three levels of invisibility, two of them entirely
in the mudlib and one of them connected to the driver. Objects may become
invisible to players or invisible to both players and wizards. Admins
may also become hidden, in which case the game driver will no longer
be able to find them using the find_player(), find_living(), present(),
and other efuns, so that the admin will be completely cut off from the rest
of the game, and therefore can edit unmolested.
   An object that is invisible cannot be seen via the look command. It
can be handled normally (picked up, dropped, sold) if a player somehow
becomes aware of its existence. It will appear in the players inventory
listing, with the note (invisible) appearing after the short description
of the item.
  If a wizard is invisible to players, then he cannot be seen by them using
the look command, nor will any messages, such as says, emotes, and movement
messages, made by the wizard be sent to players. Thus, invisibility implies
silence as well. If an invisible wizard wants to speak to a player, he
needs to become visible again first.
  Wizards may also become invisible to other wizards. In this
case, only the admins can detect the wizard and his/her messages: in
all other regards it is identical to player invisibility.
  The most advanced level of invisibility is hiddenness. Admins can
become hidden by calling the hide() efun on themselves. If this is
done, only other admins can detect them via the game driver's efuns.
Because of the way valid_hide() is coded in the master object, Root UID
objects cannot detect hidden admins: thus hidden users will not show
up on the who list, the finger list, room contents, or any other object's
output unless that object has an admin's UID. The people command, which
takes the user's EUID, will show hidden admins, but only to other
admins.
  You can make an object invisible by setting the "invisible" property
on it. The property is an integer, with 0 meaning visible, 1 meaning
invisible to players, and 2 meaning invisible to wizards. Wizards can
make themselves invisible using the "invis" command: their prompt will
change to reflect their invisibility status. Admins can hide using
the hide command.

Mobydick, 8-24-93