Transparent plastic protective cover / case for a car key fob (remote key shell).
It is a hollow, clear, hard-plastic (or rigid molded) shell shaped to fit over a typical rectangular/rounded car remote key. The small tab/protrusion at the bottom is usually for the key ring or to help seat/remove the cover. These are common aftermarket accessories sold as “key fob covers,” “remote key protectors,” or “transparent key cases.”