Imagine you know there is a ship is behind the rock or behind the arch in the middle of Northern Fjords. Now imagine all the possible ways he could go from there. He could go left or right around it and may even be at a different altitude to add as a surprise effect, he could go above or below it even if the structure allows him to or even he could go backwards and away in order to ambush from a completely different route.
I can see it being a solution to the current cloud "features" where you can sometimes see a ship, but not be able to spot it or sometimes when a ship is somehow too far away from being spotted, while clearly visible (#WaterHazard). Being able to spot a ship behind hard cover however should not be a thing, because it would negate literally every single advantage that the hiding ship has, and if it is hiding while you are already aware of his position, he can really need those advantages. In order to avoid being spotted through sheer luck the pilots would then have to be constantly moving around at extreme speeds, because those airships base speed can be rather slow. Try to avoid being spotted like that in a Pyramidion or Galleon or, god forbid, a Spire.
There is also the thing where a spotted ship stays spotted for a short period of time when line of sight is lost and so this proposed mechanic can be used to chain spot somebody indefinitely, which may be an undesired side effect.
Essentially it all comes down to a spot being a spot and our in-game characters translate the spot into information displayed on our map. If you want everybody to know that a ship is behind a rock, then you are looking at a different mechanic, a mechanic where you mark a specific location on everybody's map, telling your team that something is there.