The argument – that an alien species would interact with the universe in ways that we would not think to look for – kind of falls down on its face with the hypothetical. A sentient ant colony would notice right away that large sections of their observed universe would contain a near-infinite number of objects and activities that were not explainable as natural phenomena but could be explained as the result of conscious, artificial action: the buildings alone would give the game away. It’d be like us working on SETI and not noticing that Alpha Centauri happened to have a Dyson sphere.
At least, that’s my first take. No doubt people will pop by to tell me why I’m wrong.