Casi no se ven
This seems easy at first, but I think it means in this context: They almost can't be seen (from here). Like "casi no se ve" one almost can't see them = they almost can't be seen but I wonder about the ven and not ve - and that makes the sentence be about them and not the one doing the actual seeing.
Girl: Allí están. Los veo desde aquí. (she is on a highwire looking out in the distance) There they are. I see them from here.
Man: Que buena forma de buscar. What a great way to search (from up there)
Girl: Están en un callejón. They're in an alley(way)
Girl: Por poco no los encontramos. We almost didn't find them.
Girl: Casi no se ven
1 Answer
You are right! They almost can't be seen (from here). It´s passive voice.
Another example: No se ve nada en la oscuridad ....... Nothing (can be) seen in the darkness.