Few readers will be puzzled by the capital D in the first example and the small d (and s) in the second. “Detective MacSwain” is treated like a name, a proper noun; “detective” (like “sleuth”) is a common noun. But what form would you choose in the following examples?
Few people will accept that up means down simply because you say so in writing, not even if you’re perfectly consistent about it. Still, when it comes to editorial principles, consistency is second only to being right.
“The full names of administrative bodies are capitalized. Adjectives derived from them are usually lowercased, as are many of the generic names for such bodies when used alone . . .