From the NannyMUD documentation

LAST CHANGE

2000-12-16

NAME

        hidden_door - A secret door object.

INHERITS

	/std/door

DESCRIPTION

	A variant of the door is the /std/hidden_door.c. It is not
	mentioned in the room description when closed and the exit
	is not in the "obvious exits" list. It can't be opened with
	"open door" either, and must instead be refered to with the
	name set with set_name(). It is a good idea to have this
	name mentioned in some add_item description in the room.

	Otherwise it works as the normal door.

EXAMPLE

	Look in /examples/doors3 for an example of a hidden door.