I think this minor issue has been occurring for a long time.
In my 404 logs, I keep seeing items like "/wp-content/uploads/wpcf7_captcha/865238817.png" appear, sometimes in multiples. This particular example was triggered with 38 hits.
Browsers and even search engine spiders are looking up these dead captchas.
I'm not sure why there would be attempts to load captchas that are no longer there. This isn't a big issue, but if this could be solved, it would help clean up my 404 logs.