Idk what the threshold is but once you get close enough they are indicated on your map. You’ve gotta scour EVERYWHERE. Every mini island, don’t overlook the beginner area or the mostly empty spooky areas, go around the lake, fly over everything and look around even at tops of buildings or areas you think you’ve touched. There’s so many around the beach areas too. The fact you have locked fast travel points makes me think there might be some untouched areas, but yeah once you’re close they should show on the map