I recently started Bitcoin-Qt to receive my first payout from Slush's Pool on just over 0.1 BTC. To test how things work, I sent a small amount to one of my own addresses. I had to pay a 0.0005 fee too, and the only thing I could see in Bitcoin-Qt was "transfer to yourself" and the negative value of the fee. The balance was the same, minus the fee.

Then I went to Blockchain.info to see from what address it was sent, as I could not see that in Qt. I searched for the transaction number, and found my small transaction to another one of my addresses, but also a ~0.099 transaction to an address I do not recognize.

As I said, the balance in Qt is correct, but I don't own the unknown address, as far as I know.

This is probably a stupid question, I guess everything is as it should, but what happened?