Find where "External Links" are used in te workbook?

1) search in the workbook for ".xl*" in the formulas
2) look at the named ranges under formula -> name manager
3) check data validation
4) select the sheet and check the conditional formats

External links can appear in a number of places and are difficult to discover. The places I can think of off the top of my head you should check:

• Data | Edit Links and Data | Connections
• Formulas with external references
• Named ranges with external references
• Pivot tables references external data
• Data validation referencing external sources
• Conditional formats with external references
• Potentially hyperlinks (dont recall if these can count against external links alone)
• I am guessing things like data model, powerquery, etc.
• Hidden or very hidden (yes, very hidden is a real thing) sheet with formulas, etc. on them

Probably the best single tool to help with this, assuming you have a recent pro plus version of Office or Office 365 is the Inquire add-in and Workbook Analysis. It can report and list out external links, formulas, named ranges, etc. If you cant enable/use this then the best thing to do is work your way through possible causes 1 at a time and exhaustively check. Either way its really a manual process.

upload a file if you want and we can search it for you.