After completing the Story Mission They're Not Sleeping, you can return to the Cave. You can find it past the northern Nest of the Old Pioneer Cemetery Infestation Zone as shown in the Map below.
It will be empty even during the day but there might be several freakers roaming the parking lot on the way there so you’ll need to kill a few of them to pass safely.
Once you reach the Cave, just waltz in there and follow the path until you reach the Beacon deep inside where a recording collectible and a NERO Injector is located.