IsEmpty did not work either. Because the file path is a concatenation, Filemaker thinks there is something in the field even though the referenced image does not exist.
One would think that since the container field status is "File cannot be found" that there would be a way to extract this status.
Well there is something in the field--the file path to the missing file is still in the container so it is not empty.
I have figured out a rather clumsy way to test for this, one record at a time
I can use Export Field contents to export to the temporary folder and this will generate an Error code 100 (File is Missing).
In a looping script, I can try this using set error capture and "mark" each such record by setting a field to a value.
I don't know why I thought IsEmpty would work; I was thinking the file name would not be there; but of course it still is. Another method would be to use a fee plugin, which can test for the existence of a file (path). It comes for both Mac and Windows, from a respected company.