Comment by rsbadger

Almost all of them. A good example is Dropbox link you send to someone. I could generate this link to a private file in my Dropbox, email it you, and Bing (may) index it.

https://www.dropbox.com/s/vucien2ns8jktga/denim%20bodywarmer...

I doubt many people realise this when they email "private" links...