Sometimes there are gaps and holes when you put a map together, such as the several huge gaps seen here:

Can you draw a line of sight through these huge holes as long as it doesn't hit a wall or blocking terrain? The forest hole especially looks like characters could see each other over the big gap left in the map.

I think such holes will mostly have walls on their borders, like in the picture you posted.

No, because those "holes" have walls on their edges.

Just imagine the outdoor walls as thick forest.

No, because those "holes" have walls on their edges.

Just imagine the outdoor walls as thick forest.

Or a giant tree/rock formation/etc that would prevent you from seeing/shooting through it.