Dear @crypto.piotr

I had experimented with steemvoter about a month ago and signed up for it authorizing it to upvote certain authors. That was about a month ago and it doesn't explain the earlier unauthorized votes selling from my account.

The point I am making is that I did forget that I signed up for steemvoter and authorized it to upvote certain specified authors, until I read @achim's comment. You need to rule out such a possibility for yourself too. You need to be absolutely sure about it as otherwise you might be putting innocent people in the crosshairs.