It doesn't have to be milliseconds. Most of people don't sit and stare on the camera all day. I have multiple displays and turning camera for a second when I am looking at another display - or even the laptop one but concentrating on something on the screen, especially the lower part - would slip my attention very easily. Or I might notice something off with peripheral sight, but it is very inexact and while I turn my head to bring it into the field of sight where I have a good resolution, it could be already gone. Of course, the cover has none of these problems. If it's covered, then it's covered.