Besides the easy answer that we're looking in the wrong places by virtue of using current human technology, my take is that an "advanced" civilization capable o

Besides the easy answer that we're looking in the wrong places by virtue of using current human technology, my take is that an "advanced" civilization capable of interstellar travel will have learned to be way more efficient with energy usage. They'd have to be efficient for "advanced" energy usage to be cheap enough at the scale of a civilization instead of rare and expensive moonshot projects. So they're only undetectable as a side effect of being efficient. Not because they're hiding or have all died in a cosmic extinction.