IIRC people secretly taking upskirt photos in trains was a significant/frequent problem

So "Silent Camera" app is popular. It basically just take a screenshot from preview, but now it's enough for some use case thanks to higher resolution of screen. (Some apps are advanced maybe, not using screenshot) Some conference presentation orders people to avoid shutter sound by using silent camera app.

... who can just turn the video on and off really quick (or even write a very simple app to turn the video on for one frame).

O_O I am shocked. That's really bizarre