If you search for some brands like Nike, Adidas, etc, you may see Microsoft Bing show a coupon element in the search result snippet. This seems to be an organic feature, at least it is not on on an ad in the Bing search results...
Frank Sandtmann, a German SEO consultant, tipped me off to this and after much trial and error, I was able to replicate this and confirm it was not a browser extension or plugin.
Here is a GIF of it in action from Frank:
Here is a still image:
There is no real details on how this works but there is a disclaimer that says "This experience requires additional third party cookies."
Maybe this was announced by Microsoft a while back and I missed it or I covered it and forgot about it?
Forum discussion at Twitter.
Update: This is now officially live as of August 10th, via the Bing blog.