They appear at specific locations around the map that have vents near them. The girl on the left is near your starting house, the girl in the center is in a cave near the smallest house, the guy on the right is near the big cave with locked door. I wanted to upgrade the map so that it can expand and show you the locations, but didn't have enough time to get it in place for the patch. It will be easier to find these types of events in future updates.