For one thing, any post in a thread with DRM in it will return that thread as a search result. If you had selected show posts, it would have given you more results with specific messages.
For another, I would suggest visiting the
Dealing with Kindle for PC/Mac 1.19 and KFX in calibre for about as much help as can be given on MobileRead. For more information, check
Apprentice Alf's blog.
Detailed discussion of removing DRM is not permitted on MobileRead. It tends to attract lawyers.