Search paths. The full address is go.corp.google.com (which is simply a URL shortener), IIRC; however, I think the resolvers are also configured to respond to a bare name in a lot of cases as an optimization. They talk a little bit about corp in their BeyondCorp paper[0], which is well worth a read, and I'm speaking to ancient memory so I might be wrong these days.
[0]: http://static.googleusercontent.com/media/research.google.co...