No need to apologize for the description; your explanation is quite clear! From your description, it sounds like you might have seen a rear windshield wiper in action. Rear windshield wipers are common on SUVs and other larger vehicles, and they’re used to keep the rear window clean and improve visibility. The small rectangular section you saw might have been the wiper arm cover, which is designed to protect the wiper arm and mechanism when not in use.
While it might not be a common observation, what you described is typical behavior for rear windshield wipers. They activate automatically when you use the windshield wiper controls or when you engage reverse gear to help keep the rear window clear of dirt, rain, or snow. So, it’s not related to a rearview camera but serves a different purpose.
I hope this helps clarify what you saw! If you have any more questions or need further information, feel free to ask.