Let's imagine a remote island on Earth that was colonized five thousand years ago (and completely forgot about how they got there). They may spend a very long time sending and listening to sonar signals. However, we (the aliens from their point of view) use sonar for ourselves, not to detect intelligence. It would be very hard to detect intelligence from our use of sonars (which is similar to that of a few animals). Even if they did guess that we were intelligent, their understanding of who we are and how we live would be minuscule.

On the other hand, if they spent their energy sending ships, they would eventually find us, and they would learn phenomenally about technology, science and their place in the world, even before they reach us.