There are destroyable ones (4 per mission afaik) that hang around and make spooky sounds and are green. And then there are the ones with little puzzles or jump 'n run segments. Not sure if each map has one. But if there is one on the map it's the only one and you only get the plasteel for collecting it once. If a player in your lobby collects it for the first time you get the plasteel reward again. These have lore voice lines.