When they are in the car together sheltering from the rain after collapsing their tent, protagonist leaves girl and stepdad all wet in the front seat and steps out briefly from the car to grab some crisps or something from the trunk/boot of the car to share with them. When she gets back into the backseat, she looks as dry as a summer cat, despite the falling rain outside.
When Ann comes back to the house with Olivia to collect food, the two corpses on the sofa have disappeared. Drag marks on the ground suggest that either Ann moved them on the previous visit, or, more likely, that they have been dragged off and eaten by the infected.