I have recently been downvoted on three different SE sites in a short period of time. All of the posts had high visibility on my profile (my top answers/questions or my recent ones).
It's probably just a coincidence, but it made me wonder whether the serial downvoting detection mechanism takes multiple sites into consideration.
I read the faq, but this specific issue doesn't seem to be mentioned.
In case it does not, then I think it should. It's fairly easy to target a user active on multiple sites and get away with it.