Happened to me about a week ago. When I opened the browser, a notification popped up telling me "Your bookmarks are now being synced". There was an option to turn this off, which I did, but I have to assume that the bookmarks were already sent off to Microsoft. There's nothing remarkable in my bookmarks. I've never really found it to be a useful tool beyond a way of marking things I want to check out again soon. I do…
> All synced data is encrypted in transit over HTTPS when transferred between the browser and Microsoft servers. The synced data is also stored in an encrypted state in Microsoft servers. Sensitive data types such as addresses and passwords are further encrypted on the device before being synced.
That's pretty flowery language for "basically all your data is readable by us". It's pretty ridiculous they would do this without an explicit opt-in.