Having thoroughly explored the passages beyond our initial entry point, we backtracked out to the below-ground area we had first climbed down into. Approaching it from within the complex, we realized this was an overgrown vehicle bay - a large gate at the far end obviously opened to allow vehicles to be parked here, open to the sky but concealed below the level of the forest floor.

After a moment of nosing around, we found this small (3-4') passage in an adjacent wall.