Every game of the Thief series has some content files that are present in its folders but do not appear during play. These files are marked as unused content.
What is this?[]
Every piece of data that is included by the game developers in the release packages of the games, but is not triggered or found in any way during actual gameplay, is marked as "unused". This can include files or parts of files like audio conversation or text contents that are related to a certain mission but do not appear in it when the player plays the game it self. They are visible either in the files directories or the mission builder.
Unused Audio content[]
These can be any sorts of audio files like conversations, regular character speech ("Who is there?"), ambient sound, sound FX and so on. The snd.crf package (Thief 1, Gold and 2) contains all audio contents of the particular game, in the form of WAV files. Some of those files are actually recorded by the voice actors during the making of the game, but are not triggered in the actual play.
Unused Visual content[]
These can be any sorts of models, props, textures, NPC moving patterns etc. When a visual content appears in one mission, but not in another it is not considered unused. To be unused a content must me present at the Thief/res directory and must not appear in any of the missions during play. They can be present in a mission, but can be hidden in isolated chambers or behind impassable obstacles, but can be still visible in DromEd. Those do not include Easter Eggs as they actually can be viewed by the player in certain circumstances.
The most common and most important of the unused Visual contents are the unused Books. They are texts that can be found in the books.crf package (Thief 1, Gold and 2) and can be related to a certain mission, but do not appear in it. The books.crf contains lots of STR files. They are every piece of text that the developers have included in a mission package. Note that the text itself is not a prop, but it can be related to any visual model: books, scrolls, posters etc. Some of the texts however cannot be seen by the player when he plays a certain mission. When this occurs a check is made in DromEd if there is an actual book prop, in the particular map, which is related to that piece of text. If such is found, the exact location and properties are recorded. If there are no props that are related to a certain STR then it is recorded that the book is not visible anywhere in the mission, even in DromEd and is unused. Such pieces of text were probably ideas of the developers that never made it into the game itself.
It is good news however that those texts can still be viewed and add more story content and atmosphere to the game. All of the texts both used and not are added to the in-game text pages.