The origin of the broadcast is a ham radio in a shelter at Big John's salvage. The hatch is in a tipped train car in the junkyard but is, as mentioned in the broadcast, shut tight. The hatch can be opened by flipping the circuit breaker in the hardware store slightly to the north of the main building.
“Chuck, John - Anyone out there: my family needs your help. We're trapped in our bunker. Big John's salvage yard. Generator running the ventilation died and now the hatch - shut tight. Vacuum sealed. Please, someone...we're almost out of oxygen. The hatch is in the tipped train car. Please...start the generator. Kimmy...stay calm honey. Breathe slowly...that's it, slowly.
This has been a pre-recorded message. Message repeats in three seconds.”