Quick suggestions:
Look for any connections under the Data tab.
Also try searching EVERYTHING in the entire VBA Editor for the
string. Make sure their personal Workbook, which is a hidden
workbook, and all other hidden objects are included in the search as
well.
If there are any protected Add-In's that you can not search, turn them
off, one at a time, to see if they may be causing the issue.

Goshute