On Windows I use Mountain Duck for connecting drives to various online storage locations. Similarly if you have other apps that use the Dropbox API for access. If you upload a file via the Dropbox web interface, then the origin will not be identified as the Mac and the file in the folder would show up, but only as offline. There is an alternative approach that I think is worth exploring. Though I imagine it is something you would have to monitor and track to determine if there is anything you can reliably use. I don’t know if Dropbox triggers any file system events when it completes the initial sync that you could hook into, but that would be where I’d start looking. If I understand correctly how smart sync works, when a folder is marked as always offline, externally new files will remain offline, but new files created locally in the offline only folder will remain local as well.
