If you want to reference other files, you should use a less ambiguous way to refer to them. Like a relative path or full absolute path. The fact that that weakness is because of a half-baked feature like that actually makes me lose even more respect.

Edit: thanks for the info though, it does add some missing context.

So throw an error at runtime on that macro, most workbooks aren't the target of a macro

Whichever one has the smallest relative path to the workbook using it? How does it find the workbook if it isn't open already?