So as the title suggests, I've found a potential secret in the Stranded Tanker mission. After you destroy the oil refinery and clear the area, take a moment to explore the area.
To the right side of the area, as you come in, there will be a generator of sorts with a small red light on it.
There are a few more of these around that area with red lights on. I shot them all (That I know of) and an orange light appeared on the generator. I shot that too.
While looking around the area to see if it did anything, I noticed that next to the generator on the side of one of the things with a red light, there was a set of coordinates or something, scribbled on it. (Find the secret in the image attached)
I have no idea how to check the coordinates on the map, but if someone knows how it's worth a look. I thought it might also be streets too, but couldn't find any numbered streets, example. 3rd St or 212th St.
Maybe the x is near those two streets or to the right of the coordinates. Either way, it's interesting.