I think we're talking in circles
I agree that something completely alien to all earth life would be difficult to detect. I find that unlikely, although I know you think differently. If we are looking for a suitable temperate environment with CHNOPS chemicals, that in and of itself suggests similarity to earth life. As I said before, we should at least have some instruments that could detect earthlike microbes. I don't think it's technologically impossible to equip a small probe with an internal chamber and microscope to analyse anything captured in that chamber. And cameras and sonar sensors that could detect more advanced life forms like alien fish or plants.