Some BTC was sent back to you as change. Are you sure someone didn't sneak onto your laptop and make a purchase?

Nobody was at (my) home that could have done that. At that time being i wasn't at home, so that even scares me more.
So, apparently i got hacked (or backdoored) and now i've lost 0.5602 bitcoins from my wallet since 21:32 GMT +1, the 31st of March.
I have NO idea how this could have happend as that i have full AV,
The recipient's address is "1FmMasfuEE6wtpzdWiVnDynfJ5tyyVbhP3".
Can anyone give me some advice / help here?
can you be more precise?
what is your bitcoin client, which one is your address
Bitcoin-QT; 1AV1CdiYq2uSUwJzdu5UmLAD8GdQxRwBhE
Just ran a check with Hitman PRO; it flagged all my cgminer exe's as malware, but i've been using them for about half a year and nothing like this has ever happened in the past.