Empty merge fields introduce characters that cannot be found
When you perform a data merge and create a new merged document InDesign adds hidden characters where the empty data field would be. These characters cannot be found using a standard search. I have not tried GREP.
In my case I was working on business cards. Some records have no mobile number so there is a blank line in the merged document, apart from the M prefix that would normally indicate a mobile number. I wanted to remove all those lines so I selected one, from the M at the beginning to the paragraph break at the end. I pasted that into the Find field, left the Change field blank, then clicked Change All. Nothing was changed.
I replaced the odd characters in the Find filed with a wild card, so now I was searching for "M^t^?^p". That’s an M followed by a tab, followed by any character, followed by a return. Nothing was found. I tried two wildcards, then three wildcards. Nothing was found.
InDesign’s merged document contained some odd character that cannot be found by any means, even with wildcards.