Perhaps the integration will extend to triggering a search on the back end.

I suppose any company that is giving all their encrypted data to Dropbox to begin with may be OK with it. But most companies are already sketched out by the mere fact that their data is accessible to anyone outside the company.

In any event, if they were to index and provide search as a service as well, I wouldn't think it's something they do quietly. It would most likely include it's own huge marketing campaign.

Could Dropbox detect repeated access patterns from the same process, and/or whitelist processes as known "searchers," and start returning blank files? This seems like the kind of problem only a unicorn would dare to tackle, but as luck would have it...?

So search is broken in these directories?

That seems like a lousy tradeoff.

You want to save space by not having data on your local system but use a local search to look in the contents of files not on that system? You can't have your cake and eat it too.

Sure you can: index it once, and then stream it forevermore.

except remote contents can change

Store a hash or checksum in the index and also allow the remote API to return the hash/checksum to see if the file has changed

I believe this is not the case here. In order for the files to start taking space on your drive you would actually need to right click that folder and choose "Save a local copy".