Bunkers are underground shelters hidden at most ambush camps. Clear the camp and open the hatch to get a bed, a save point and a locker. Many also give a crafting recipe and a regional map.
Clear every hostile at the ambush camp first. Then look for a hatch in or near the camp.
A cleared bunker becomes a safehouse. That gives you a spot to sleep and save deep in hostile ground.
Farewell
Farewell is the Oregon city where Deacon, Sarah and the Mongrels lived before the outbreak. It falls at the very start of the story and only appears in flashbacks, so you never ride its streets in free roam.
Pinnacles Camp
Pinnacles Camp is a cut survivor camp from the removed Pinnacles region, near Crater Lake. It would have been the sixth encampment, and you cannot visit it in the released game.