It’s most likely an autofocus-assist pattern from another camera, usually a stills camera or flash unit in the audience—not the video camera itself.

Many cameras/projected AF-assist systems cast a red grid or patterned light onto the subject to help the camera focus in low light or low-detail scenes. If someone nearby uses that feature while taking photos, the pattern can briefly appear in the recorded video.

So the artifact is not a YouTube issue or a sensor fault; it’s external light falling on the speaker during filming. In a talk or lecture setting, it can be distracting and is often considered poor etiquette if used unnecessarily.