It's a new address, owned by your own client. bitcoin-qt uses a previous incoming transaction to spend on a new transaction. If you send less, you will receive change in a new address. If a hacker got access to your wallet, it would be empty.
to spend on your last transaction. The change (849 mBTC) got sent to a new address, owned by your own client. This is why your QT client displays the correct balance, but the blockchain of your 'main' wallet address doesn't.