Search for empty fields

For many, many years I thought there was no way to search for empty fields in documents. I thought it was not possible, but it turns out it is:

The answer is to use the search term "NOT [fieldname] is present", of course substituting "fieldname" with the actual name for your application.

Source: Alan Lepofsky's post Searching for blank fields.

Thanks to @arnecoomans for the tip!