Somewhere - somewhere, this question has been asked to JK herself and she did answer it (I will try to look it up for you for the real answer).

My answer - I do know that JK has said that owls just don't automatically know where to deliver, so there is the distinct possibilty that a Ministry owl would be unable to find Sirius (Harry was using 'trusted' owls and he also had the ability to use the birds that Sirius sent to him - Sirius could have ensured by magic that only certain birds could have found him). It is like saying Sirius had made himself 'unplottable' - like Hogwarts, and 12 Grimmauld Place etc - only those with permission to know could find him.

Also, how would the Ministry follow the owl? There doesn't seem to be such a thing as a tracking device in the wizarding world - or Harry could trace Hedwig wherever she went, and Dumbledore could track Snape if he felt like it.

Magic is powerful - something as simple as a charm to protect you would prevent a ministry owl finding you if you didn't want to be found. After all - Sirius was innocent, and the Ministry would have killed him for doing nothing wrong.

Ive thought about that before, but just kind of put it down to the ministry are too dumb to think of it!

Hi there, can I just ask you a question that you may be able to answer for me! I'm reading Harry Potter again and have just come across something in my mind that I can't seem to figure out! I know how good you are with analysing everything and knowing the in's and out's of Harry Potter. So here goes, Harry is in contact with Sirius in the Goblet of Fire through Owls, and as the ministry of magic are looking for Sirius, he has told Harry to be careful what he says in the letters in case the owls are intercepted, Harry doesn't know where Sirius is but the owls can find the person the letters need to be delivered to, how come the Ministry of magic didn't just send a letter with an owl to Sirius and follow the owl to find the location he has been hiding in? This probably has such an easy answer but it's been driving me insane! Sorry for bothering you with this question!