Kelvin worked for Dharma, I guess the question is how much did he learn from 'legitimate' means and how much did he learn from snooping around? Certainly the Pearl hatch was a good hike away, as was the staff. There would likely be some way to have pre-knowledge of other hatches, woudl be very difficult (dangerous) to wander aimlessly just looking to see what is out there.
If he was that good a spook and had time on his hands, perhaps he knew even more about where the hostiles went and what they did than was put on the map.