The Apollo Statue
It can be easy to miss if you're doing a quick read through, but Apollo's statue is always on stage. Apollo never makes an appearance himself, and the gods aren't directly involved in the play – and yet, there's his statue, always hovering on the edge of the action. The point is that the gods are indirectly involved in the plot; remember that Orestes is sent to Mycenae in the first place because Apollo's oracle told him to go. In addition, Clytemnestra's dream has is probably sent by the gods, at least in her mind. The gods are somehow present all the time, and the statue serves as a constant reminder.