About an hour ago I was on Torchbearer server in Port Oakes when I saw the sky change color and then saw the event announcement "Zombie Invasion in Port Oakes." My teammate and I waited for about 15 minutes but the zombies never showed up.
A few points:
1) No, there were no other instances of Port Oakes, at least not according to the Rogue Isles Ferry (I checked).
2) Yes, we were on the ground.
3) No, we were not under anything.
4) We were out in the middle of a street well away from any arbiter drones or any other NPCs.
5) We did not have any active stealth powers.
6) Also, other people in the zone also were not seeing any zombies.
7) After waiting a long time I flew around the zone for a bit and saw the normal NPCs were still standing around, but no zombies anywhere.