Gmail often fetches the image from Google’s network shortly after a message arrives, before anyone looks at it. Apple Mail loads it through Apple’s servers in the background. Company filters open new mail to check it. All of this can happen within seconds.
These events are listed, but they never fire an alert, and a single open does not send a notification anyway. A later open from a mail program on a home, mobile or office network is a better sign of human activity, though still only “possible”.