My personal theory is it's something to do with Dropbox's Explorer shell extension. (But I really don't know enough to day for sure. Still, if the shell extens

My personal theory is it's something to do with Dropbox's Explorer shell extension. (But I really don't know enough to day for sure. Still, if the shell extension mechanism is such that you get a file or list of files, it'd need to figure out whether they're in a Dropbox folder or not and play with the icons.)