I went to download Unity again and this little pop-up appeared in the corner. It looked a bit suss so I hovered the link and it appears to be some “siteintercept” URL.
I don’t even think Unity has ads on their site, and I have Adblock Plus, so it’s either a survey actually sponsored by Unity (albeit dodgy looking), or it’s not meant to be there.
Just a heads up in case it’s on your end, not mine.