Hi Bob M,
In order to use catalog Search for this field, we will need to update which fields Photo Mechanic Plus indexes for the catalog, and as such this thread has been moved to Feature Requests.
In the meantime, as a workaround, you can do this in the Find panel for any Contact Sheet (including ones containing multiple folders) by restricting the Find to "Metadata" and the "Alt Text" field and leaving the query field blank. The Find command will result in those files with empty Alt Text files being selected. You can then filter the contact sheet to only show selected. For things like this I also like to save my selection list in case I inadvertently deselect something. If you have a significant number of files, you may need to do this in batches. But for hundreds of files, using the Find command could help you.
Mick