A proxy request can be close to a real open, because Gmail often fetches the image when the message is shown, but it is not proof. It also hides the reader’s network and location, and a later reopening may not create a new request.
Clearer machine cases get their own rules: R-OPEN-GMAIL-PREFETCH when Gmail declares that it is fetching in advance, and R-OPEN-GOOGLE-SCAN when an image is fetched from Google’s network shortly after sending. None of these trigger a notification. For a stronger signal, look at clicks and replies.