Close workbook in other instance(?) of excel

I want to close a workbook programatically but the workbook was created was created by another program.

I can close other workbooks alongside my main spreadsheet (where the code) is. However the one I am trying to close seems to be running in another excel instance(?) and the workbook doesn't appear in the windows list.

Any help would be appreciated.

Please update your profile so someone will know what version of excel you are using.
Just to clarify, why are other workbooks being opened in other instances?