[microtonal]

Hey, this file matches one that the FBI/NSA is looking for via NSL.

Or why not more mundane: another user shares a file with you. Dropbox knows that you already have the same file somewhere on your filesystem outside the Dropbox folder (or a partial match). It doesn't have to transfer that data to you.

But I agree with those saying it's probably a result of some implementation issue (Finder extension or working around some shortcoming in monitoring just the Dropbox directory).

[jjarmoc]

This is pretty close to how their deduplication used to work, and isn't too different than how rsync works.

Of course, this gave rise to the ability to transfer files (even non-public files) quickly between Dropbox accounts provided knowledge of the hashes of its chunks, and Dropbox has since changed their deduplication. See https://github.com/driverdan/dropship