I noticed Tags in the Finder, too. I just turned off Spotlight indexing in all my databases, and managed to delete all the tags in Finder except five really stubborn ones – nothing I tried makes them disappear. And the odd thing is they are definitely Tags once generated from within DevonThink, but they no longer exist in the databases. Do they reside in a library file somewhere?
Tags are only exported to indexed files/folders or the Spotlight index (~/Library/Metadata/com.devon-technologies.think3). Maybe these files/folders were tagged in the past but are not part of a database anymore?
Sharing the experience: If you used to copy the same files/folders from the Finder to external hard disk as a way to back up during the DT2 period, this could happen. The tags that you see could be coming from the older/very old backup before DT3. You can trace through the location of those files under the tags you see in the Finder.