Engineers create ‘lifelike’ material with artificial metabolism 7 years ago
I'm surprised no one mentioned "The Invincible" by Stanislaw Lem. Basic idea of the story is exactly what you're describing, "self-replicating system", with "creators long forgotten".
I recommend this novel, among with many others ("Fiasco", "Eden" and, of course, "Solaris"), for anyone who likes good sci-fi about "first contact".