Our own planet has shown signs of life for ~a billion years or more -- the same features we ourselves are only now learning to detect in other star systems have long been apparent to all who know what to look for. Who's to say how long ago some other civilisation might have spotted us? It's not like WE are going to ignore other life-providing planets simply because there are no radio signals emanating from it. If we can conclude that another planet, however far away, harbours some kind of life that alone will place it quite high on our list of planets to keep a close eye on. I don't think it's at all unreasonable to assume that ours would hold a similar place for others out there.
It's possible, IOW, that Earth was spotted long before any large mammals -- or even reptiles -- were crawling around on its surface.