Reasons why Dropbox could be accessing files it should not be accessing

I have encountered the same problem. Here is the answer by Dropbox support:

we use an Explorer dll (%APPDATA%\Dropbox\bin\DropboxExt.xx.dll and %APPDATA%\Dropbox\bin\DropboxExt64.xx.dll) to draw syncing icons on files and present context menus when you right click. When you open an Explorer window, we need to check if those files are in the Dropbox folder so we can render the icons. In order to do this, the dll lists files in the open Explorer windows and asks Dropbox if there's anything to draw on those files. That way we can decide if files that are in Libraries, or possibly symlinked from the Dropbox folder, need syncing icons.