"I have keys, as you know, with which I can open any chamber or cabinet in Paris" (36). Whoa. That's G— the "prefect" (head of the police) talking. Never mind suspending habeus corpus; this is some serious violation of civil liberties.
G—'s keys symbolize the creepy surveillance tactics of the police—that is, official law enforcement—but they also serve as a symbol of exactly the wrong kind of inquiry. G— works through attrition. If he just looks everywhere, he thinks, and uses every single one of those keys, then he's sure to find the letter eventually.
Of course, he's wrong. All the access in the world doesn't help if you don't know how to look. Notice that while G—'s style of detection is symbolized by a useful prop (the keys), Dupin's is symbolized by something a lot less useful (and a lot more cancerous): the pipes.