This exact same thing happened to me, just a few days after I received 2 small donations into each of my Multibit wallets. Each wallet received 2 donations of 1 satoshi at ONE of it's many addresses (turned out to be spam, sending addresses started with 1Enjoy and 1Sochi). Meaning, whatever it was that made those donations knew what addresses were in what wallet; out of nearly 50 addresses, it chose one from each wallet, including one wallet with 1 address! Meanwhile, my coinbase addresses (not used for anything but buying coin and sending it out) received nothing. Seems as if something had knowledge of my Multibit accounts.

All but one was encrypted. Only the unencrypted one got stolen. Yes, I know, I know...but this was an account with not much in it, and it was on a Mac that always has the process list monitored like 20 times a day (I work in IT), and regular virus scans. I'm running another one now, but expect to find nothing. It's a laptop that's ALWAYS under my physical control, or locked up at home. I can only believe that if the security breach was on my end, it has to be Multibit itself, not something I did.

Here's the transaction that stole 0.086 BTC from me (thank god a small amount), and some from another account for a total of about 0.12 BTC: https://blockchain.info/tx/04c06700f24e48b9c4dc83f440e7bd15459fe685e6a34d2578c57df9f9816bb4

Any idea what I should do? I was thinking of investing more in Bitcoin, but I don't trust online wallets for just this reason, now it looks like local wallets are vulnerable too! How the hell did somebody get the private key of my Multibit address? My only other thought is if something got intercepted in transmission; when making a transaction from a local account like Multibit, is your private key ever transmitted (un)encrypted, or just the public address? Sorry, I know a bit about Bitcoin, but not everything.