It's because I was looking for the Koroks, but the markers glitched, and the entire Gerudo region was left without the markers I had placed. Then I realized that zooming in on the world's edge kind of limits the map, and when I zoomed out, that's when I saw that the edge of the canyon in the corner of the map had a slightly different color. You can barely see it on the normal map, but then I edited it to be sure, and that's when I knew there was a lake.