I have known existing files that Mac Finder will not find if they are in a DropBox folder. The files are treated as if they are nonexistent. If I move such a file outside of DropBox, Mac Finder finds it. If I set Mac Finder to search "My Mac" it is more likely to find the files, though not all of them. If I set Mac Finder to search "DropBox" it doesn't find any of them.