This is not an advertisement or vpaid script what sends viewable impression but IMA SDK.
It is waiting 10 second to get 404 error on video file.
While waiting for video file, 2 seconds after AdLoaded event was issued by vpaid creative, IMA SDK sends viewable_impression without any signal (except AdLoaded of course) from vpaid creative.
Should this viewable_impression be send by IMA SDK without any of impression or start signals received from vpaid script ?
Tested also on IMA SDK test suite with the same effect.
You can track sending viewable_impression on network panel in developers tools.