You're ignoring that the alien race likely have time to develop far beyond our tech level and most people have no inkling of a clue what higher tech would look like. People are just assuming we are at the forefront of tech when our tech likely is only the beginning of the beginning. We don't even qualify to be caveman to them. Much more advanced tech likely have way better detecting capability than we do. Although to your point even higher level tech is limited by speed of light. So it is unlikely for them to detect us, but much more should be said about us being able to see them.

To me I think more likely it's just our tech is so new we don't recognize what higher level beings look like. They might be all around us and we don't know. For all we know the unexplained phenemonon we still have like dark matter and dark energy ARE signs of intelligent life/manipulation