About Alejandro González Medina Expertise I can answer many questions about detective literature, mainly about authors of the Golden Age of detective novel (e.g. Ellery Queen, sir Arthur Conan Doyle, Agatha Christie, John Dickson Carr...). When I say "many questions" I mean styles, main editorials, old editions, heroes and villains, mistakes of famous novelists... all about the world of fictional sleuths.
Experience I´ve read the complete bibliography of Agatha Christie, Dickson Carr and Ellery Queen and I have consulted a lot of essays about police procedurals, forensic medicine, toxicology, balistics... Actually, I´m designing a web page in spanish about this kind of literature, and, in my spare time, I write short detective stories.
Question Sometimes, I'm not sure about just what information an omniescent third person point of view can say about a thought. For example, If I'm writing in the third person, and say the girl at the library desk was quite pretty, can the narrator point out that years ago she had been severely burned and was not at all pretty for m many years? Is there a handy rule that governs this kind of thing, where the narrator and the third person speaker have differing opinions?
Answer Omniescent third person can say EVERYTHING he/she wants. There´s no restriction. In a book about writing mystery fiction (and that information can be used in other types of works)), H. R. F. Keating talks about the "angel above the shoulders". When you write, you have to think in that way: think that you are an angel that is able to see everything (including feelings and projects-in-mind).
But... be careful! Sometimes, specially when you want to hide some facts for the finishing stroke, you must enter in your characters´ mind only sometimes. There´s no rules for that but your intuition.
Taking your example, is completely factible a contradiction between the main character´s opinion and omniescent third person. There´s more... IT´S A VERY GOOD IDEA to confront the opinion of a character and the reality see by an omnipresent force.
Good luck, you are in the correct track:
Alejandro
PD: ... and excuse me for my delaying. It´s not my fault, there has been a problem with the email for Question Pendind notification. Sorry!!