my understanding was that it would be an electromagnetic signature (ie we could detect infrared radiation from the heat generated as a byproduct of metabolism,

my understanding was that it would be an electromagnetic signature (ie we could detect infrared radiation from the heat generated as a byproduct of metabolism, or, at least, whatever energy transformations they perform in their technology, once we're talking about technological civilizations). that's what would be near-impossible to conceal (a biosphere might also be a marker, but that might be concealable somehow). and if it's true that metabolism would be detectable, then there couldn't really be such a thing as 'quiet' aliens anyway, that was my question.