Symptoms
When a user makes a Google search containing YouTube (such as “youtube facebook channel” or “youtube cat videos”) Google will return video previews along with the typical links/URLs.
If the browser is Chrome, when the user hovers over these video previews Google will pre-load the video into the browser’s cache with a URI like “encrypted-vbtn0.gstatic.com”. Then if the user clicks on the video, we don’t see a youtube.com request (and can’t block it).
Resolution
In order to prevent this, do not create any white-list rule for gstatic.com or *.gstatic.com
If you see this behavior, it is recommended that you perform a list audit for these URI’s and assess if updating these URIs will resolve this.