Solar Sails, Generational and Cryogenic Ships: We’d be able to see the craft slowly approaching us. Covering it up would be increasingly impossible as it approaches us because as the comrade from before said, telescopes are readily available technology. Even if there was a coverup of an international scale that had the support of every single institution, it would require dealing with thousands of amateur astronomers and utterly subverting scientific peer-review.

Wow: The Signal has never been repeated. It could genuinely be aliens who sent it by accident. Other more sensitive radio telescopes didn’t detect it though. The discoverer believes that while it isn’t impossible that the signal is extraterrestrial in origin it could also be some nonsense done on Earth. The signal band is protected but it could be interference from a close-by frequency or someone with a powerful transmitter making the funniest pirate radio broadcast ever.

Asteroid: If they were trying to communicate with their folks back home, there’s a good chance we’d pick it up. Furthermore, maintaining a livable space station on an asteroid will generate waste heat and such. Our astronomers would probably notice such an anomaly.