"How did Google find out the other companies were being targeted?"
Google runs data centers, and probably sees a lot more traffic than for its own services. Also, who is to say these companies are not using Google products (Gmail, Docs, etc.)? In other words, I think Google might be in a position to know.
Yes but that's all traffic to and from its datacenters. I don't touch Google if I request a page from bofa.com, e.g., except maybe through their public dns.
Google runs data centers, and probably sees a lot more traffic than for its own services. Also, who is to say these companies are not using Google products (Gmail, Docs, etc.)? In other words, I think Google might be in a position to know.