Mary Lacy might think there really is undisclosed evidence in there - and if there *really* is, it's not a good idea to let it out.
It will certainly be interesting to see if it stays sealed.
Lacy's motion to seal seemed to imply that she would agree to unsealing once Karr was charged. Now that she's not charging him, will she still be willing to unseal?
What was John Fernie's occupation?
Think about the yearbook. That's the same type of mind that wrote the ransom note. Plenty of sarcasm, exclamation marks and commands.