I work in Oakland, right across the hollow were you were at. This is a Lifeflight chopper. They often circle Flagstaff Hill (where you were) when the landing pad at UPMC already has a bird down. Once the pad is clear, they land with their patient and offload. I see them do this often from my office window. This is not an unusual thing to see, they were not interested in what you were all doing in anyway I am sure. By you title, it seems you are implying they were in some ominous way.
