I found one of my iS images being offered as a free download on pixgood dot com (no direct link to avoid boosting SEO).
Like so many of the others, it would be a PITA to check for your images there; I happened to find mine on a Google reverse image search.
I have contacted CR at iS.
It is unwatermarked, so presumably lifted from the site of a legitimate buyer.

It reminds me this thread http://www.microstockgroup.com/image-sleuth/hi-res-unwatermarked-images-on-imgkid%28dot%29com/