Check out family land, even extended family members. Sometimes around here, a family member will ask permission to hunt on my stepfathers land and he always lets them.
Next see if Sandy and/or Casey hunt or fish. If they have a hunting lease, that could be a big wide area that could be searched
Abandoned country houses/farmlands
And does anyone know if Sandy ever worked in construction? Maybe helped pour cement for foundations? Even if he didn't work in the area they live in, if he worked out of town, he could've traveled to a job with poor Erica thrown in the back of a truck, or even as tiny as she is, put into one of those large rubbermaid totes.