It’s a key switch (more than likely) for an alarm system that no longer exists; key switches were fairly common in the days before keypads to arm/disarm. The back of the door where that hole is used to have wires going to it, and if you were to put a wrench on the two flats it would unscrew if you wanted to remove it and have the hole filled. Right now it’s not connected to anything, so nothing to worry about. Hope that helps!