windandthestars wants to know: When do you capitalize canon-specific proper nouns? (ex. the Sanctuary)
Technically, all proper nouns are capitalized, so the question within the question is, "When is a noun a proper noun?"
Building names are proper nouns, as are business names and the names of associations and groups. In the example of Sanctuary, the larger organization of people helping abnormals is called the Sanctuary Network, and both the group of people headed by Dr. Magnus and the building they work out of are called the Sanctuary. The other associations/buildings around the world are also capitalized as names, e.g., Tokyo Sanctuary.
If you were using the term in the generic sense, it wouldn't be capitalized:
As the escaping abnormals fled from their enslavers, they could only hope to find sanctuary.However, because the word is so meaningful within this fandom, it's better to use a synonym like refuge unless you want to deliberately capitalize on the wordplay.
In the DC Universe, the Daily Planet is a building in Metropolis, a business in that building that employs Clark and Lois, and the title of that business's product, Metropolis's major newspaper. The capitalization holds for the nickname, as well: If Superman saves the Planet, he has foiled an attack against the building. If Superman saves the planet, it's the planet in the more general sense—Earth.
Alfred doesn't mind at all if wildlife biologists go looking for a bat cave, as long as they don't find the Bat Cave. Meanwhile, Bruce Wayne makes sure that the enterprises Wayne Enterprises is engaged in are successful (although you might want to say activities or deals to avoid redundancy), and Lex Luthor keeps an iron control over his corporation, Luthor Corp.
In short, the rule is to capitalize the names and nicknames of buildings, businesses, and associations, and be sensitive to word choices that have extra resonance in your fandom.