My container (used for land-based storage) has obviously seen travel, and has nothing beyond a couple corner air vents. None of the other containers I'm familiar without here (they're quite popular for storage) have anything of the sort. That valve is a nice idea, but doesn't really exist in most of the fleet of containers on the ocean.
They are, indeed, a hazard for other boats, though they generally won't "lurk below the surface." Either they're floating from buoyancy of what's in them, or they sink. The increasing water pressure as you descend makes "floating below the surface" a particularly unstable place.
Perhaps some new ones do, but generally most containers have nothing of the sort:
"Depending on whether they are full or empty, and on the nature of the cargo inside, containers may float at the surface for several days or weeks prior to sinking. Containers are not generally entirely watertight; while an empty container is likely to sink due to water ingress, a full container will likely float until air trapped in the cargo has escaped.’"
They are, indeed, a hazard for other boats, though they generally won't "lurk below the surface." Either they're floating from buoyancy of what's in them, or they sink. The increasing water pressure as you descend makes "floating below the surface" a particularly unstable place.