Perhaps they periodically send probes like Oumuamua that report on nearby systems.

We can observe a lot more in the cosmos than we can physically reach. We are mapping out and tracking far more stars and planets than we could hope to reach for a long time. Even within our solar system, we have gained and will gain much knowledge of the outer planets from probes well before we set foot on them. I would expect this property to hold for other civilizations, it's just much easier to observe far away things than to transport complex life forms to them.