The main idea of the second paragraph of the text "La carta robada" is:
B) Búsqueda de pistas misteriosas
In this paragraph, the text describes how the police searched thoroughly for clues in the minister's apartment but found nothing significant. This sets the stage for the mystery element of the story as they struggle to uncover any leads or hints that could help them solve the case of the stolen letter. The focus is on the detectives' efforts to find mysterious and elusive clues that might lead them to the missing document.