Just tried it. Created almost totally random patterns on the screen in a silent room.

Many webcam mics have auto-gain enabled, so having it on in a silent room will cause it to crank the gain to the point that the noise floor becomes audible (or in this case, visible). You might want to try recording a sound clip to see if the same effect occurs (i.e. you can hear the static fade out when louder sounds like speech hit the sensor after a period of auto-compensated silence)

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