He could have been tricked. It isn't so difficult to manufacture evidence of a false death.

People say that, but I haven't seen any evidence that supports it.

[Conan fails at deductions related to his personal life. He can read murder motivations perfectly, but he can't tell when a girl has a crush on him. And he always forgets his birthday. Well, he was living on his own since the age of 14. It isn't really surprising he doesn't care much about celebrating it.]