image blocked by CSP policy even though the image is never requested

The following CSP directive violation is reported:

        (document-uri) =>
        (referrer) =>
        (violated-directive) => img-src
        (effective-directive) => img-src
        (original-policy) => img-src 'self' https://various.uris <but NOT>
        (disposition) => enforce
        (blocked-uri) =>
        (line-number) => 4
        (column-number) => 31488
        (source-file) =>
        (status-code) => 0
        (script-sample) => 

If I understand correctly, this means that a visitor to my-page was referred from Google search and in the course of viewing that page the file was blocked. But I cannot understand how there was any attempt to display that file. It is certainly not directly referenced anywhere in my-page. The script is not corrupted and makes no reference to that file. Nor does it appear in any other file or database for my site.

So, how can I figure out why there was any attempt to display that file in the first place?