I can’t find the recent news article but this story[1] is the same in essence: Bing starts indexing web pages it impossibly can know about unless some Microsoft software sends visited URLs to Bing. In the news I remembered it caused some sensitive leaks because company secrets, only obscured by unguessable URLs, were suddenly listed on Bing.

[1] https://answers.microsoft.com/en-us/bing/forum/bing_websearc...