Yes—this is most likely the webcam’s internal image processing, not the sensor somehow becoming noise-free.

Webcams and IP cameras often apply aggressive processing to reduce visible noise and save bandwidth. A common approach is to threshold very low pixel values and force them to pure black, sometimes along with built-in noise reduction. So even though the sensor still has noise, the camera’s processor may be zeroing out those near-black values before you ever see the image.

That’s why a fully covered frame can come out as perfectly black, while noise appears only where some light lifts the signal above the processing threshold.

In practice, this means manual dark-frame capture is usually not useful with this kind of camera. Many sensors/cameras are already factory-calibrated for dark-current offsets, and consumer webcams typically don’t give raw enough output for proper dark-frame subtraction.

If low-light timelapse noise is the problem, you’ll likely get better results from video/timelapse noise-reduction software or from a camera that can output less-processed data.