I'm now going to be one of those annoying people who points out mistakes in Harry Potter. In OotP, 12 Grimmauld place is so heavily disguised and hidden that you can only get in there if Dumbledore personally tells you where it is. And yet Harry managed to send Hedwig there before he knew where it was by telling her to find Sirius.

So why doesn't the ministry just send an owl to Sirius and follow the owl to find out where he is?