I looked at the docket and even consulted with another attorney here in our office (who used to clerk for a federal judge). I don't know what's going on, but the only other sealed documents in the docket are summons, which probably contain Madigan's home address. This thing appears to be a letter, but there's no indication of who sent the letter, or what personal information it might contain. If anyone wants to make a quick call to the court, the clerk should at least be able to provide some general information on what it is and why it's sealed.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.