I think that it is very likely that numerous civilizations have seeded the star systems of this galaxy and others with probes that sit and watch for interesting

I think that it is very likely that numerous civilizations have seeded the star systems of this galaxy and others with probes that sit and watch for interesting/threatening things. The probes have arrived far before humans or even life existed on earth and have been waiting and watching ever since, with some of them possible reporting back to their creators on the state of earth while others still watching without yet sending a message because the state of earth hasn't reached some threshold. As for if these signals have made it back to the probes' creators yet, I consider that unlikely due to the scale of the universe.