Signals from a Kepler Planet

Frontier physicist Zhou Ze accidentally discovers a mysterious signal from the planet Kepler, leaving the whole world astonished and intrigued. For the first time, humanity finds intelligent signals amid the desolate cosmos. After three months of decoding, they finally learn the meaning of the message from Kepler: Hello, Earth! We hope you received this signal. We have been watching you for three thousand years.

After the initial excitement and exhilaration, Zhou Ze realizes that no more signals are coming. Earth keeps sending messages, but never receives a reply.

The world's leading physicists are gripped by panic. What does it mean that the other side has been watching us for three thousand years? This is certainly no friendly signal. It sounds more like the low growl of an animal about to attack its prey.

What does it mean? Has Earth fallen into a crisis that was prepared long ago?