I can’t think of anything compelling that would have been left in the parking lot giving rise to a search, given that they had so little — and apparently nothing was recovered from the parking lot or at any point in the trail.
Ellen had no phone of her own, neither had hats or extra water despite a ~10L deficiency for the intended route, and Oski had no water bowl.
I know these are the reasons why we all find the whole situation so odd. No water bowl for Oski. So little water. I was thinking perhaps keys (and they found them in the parking lot or possibly had a second set if they thought they lost them but had not they eventually realized as it got hotter and hotter.)