I've only found the Family grave, and I have theorized that because the "Special Encounters" In the original games were effected by perks, but not by location, and that this game as multiple characters running around that they have a "Set" area where they MAY spawn. The Themepark one seems to be an obvious test of that, and I've had people damn near pin point the area where I got the Family Grave, however that was 2 wipes ago, and at that point the perks did at least Mention they helped with 'special encounter' finding, now they do not.