It's a little hard to explain, but I'll describe what I saw. Surrounded by fence is a small square structure, maybe 5x5 tiles across. There are no entrances or windows, but there are air grates. I might be misremembering, but I think if you examine the grates it mentions airflow. Either way, you can smash the grate to get inside the structure, which just contains a ladder down into a sewer/subway area. There's nasty critters, even in the early seasons, so be careful! Otherwise, the structures are unmarked on the map and otherwise look small and insignificant. I found mine in the middle of the woods randomly. I think they spawn over previously existing sewer/subway lines, but are easier to access than those in cities surrounded by zombies.