You are entirely correct, hence my assumption. I think non-expansive civilizations are out of question as it's far more likely that we are never able to detect

You are entirely correct, hence my assumption. I think non-expansive civilizations are out of question as it's far more likely that we are never able to detect them even if they exist. The energy trail is more debatable, but that necessarily needs a violation of the second law of thermodynamics, so it still sounds like a reasonable assumption to make.