Find/Break Links not finding external reference

Hello,

I have a spreadsheet that has started to say there is an external reference in it, after some recent updates. I have checked the changes made, but cannot find any external XLS(X) reference - either in formulas or in conditional formatting.

When I run the VBA routine suggested on the 'How to find links...' page (https://www.extendoffice.com/documents/excel/953-excel-list-all-links.html), it generates a new sheet with a single external reference (to a valid spreadsheet document) - but it does not tell me where that link can be found.

I then installed the 'Kutools for Excel' application. When I run the Find Links function, as suggested on this help page, it says there are no formulas in the worksheet.
I do not know if this means it cannot find an external link or actually thinks the worksheet has no formulas in it at all (which I know would be false).

Can anyone suggest how I can find this mysterious link, so I can remove it?

Thanks,

Brian.