I literally saw my client a few moments after it happened (it was open) so how this happened is odd!
My actual User account that has been stolen from is
NXT
16821029889165561706
I don't have any idea how this may have happened either. Just wanted to confirm, at the moment the theft happened your client was running and you had the browser window opened, and your account was unlocked (you were seeing your balance and the "send money" arrow), is that all correct?
Just trying to differentiate the possibilities, whether the hacker obtained you password via brute-force or some other way and initiated the transaction from another machine, or somehow your own machine was tricked to initiate the transaction.
And you were running 0.4.8 at the time, right? I added the second check for secret phrase before send money exactly to increase security, so that even if you account is unlocked in the browser you still need to enter your password again.
Another question, did you generate your random-looking password using some software - password manager, online service, or created it manually by typing at random?