When one of the space vehicles was doing a swing past Earth, for speed, Carl Sagan asked NASA to turn their equipment to the Earth and see what they could detec

When one of the space vehicles was doing a swing past Earth, for speed, Carl Sagan asked NASA to turn their equipment to the Earth and see what they could detect. The surprising outcome was that the tools aboard that particular probe was unable to determine life on Earth, even when swinging in orbit around it. Mind you, that wasn't it's original mission plan anyway. But it shows that you have to look for very specific things, and as of yet we have not detected life on Earth from Space (other than our satellites, but that's a given).