Without knowing all the details, generally shipping containers transporting humans, there's a overwhelming smell that emanates if they're been kept inside for over a day.

My dad was in container shipping, every once in a while there was a call from the port about a badly smelling container. They knew there was a human inside, just not sure if they're alive or, dead. Authorities were called and they'd separate the container, open it up and see if there's a body or, somebody(s) living with buckets for their waste.

Don't drivers of cargo check the load for content and if it's secure before starting the haul?

Looks to be a freezer trailer. It wasn't the weather that froze them.