Nobody noticed because everything on the open net is constantly being slammed by bots.
This was just more bots.
That explains why nobody noticed on the internet who was being used in/targeted by this attack, but it doesn't explain why OpenAI wouldn't have noticed traffic getting out of their "sandbox" when they knew it wasn't supposed to.
One website I'm responsible for is getting 500 requests per second from detected bots. It's quite ridiculous now.