You should try this on a recent (last 4-5 years or so) Macbook! There is a hardware timer that forces the LED on for at least 3 seconds even if the camera only grabs a single frame.

I meant that by the time you notice the led, if you do, the picture was already taken and probably sent over the wire.

IIRC, it takes about half a second for the camera to initialize from power-off and take a photo. The flash this creates is pretty noticeable.

Besides, on current versions of macOS, you need to grant permissions to an app before it can use the camera.