If they are guilty, it probably isn’t their first time. Cops probably need to bring in some ground penetrating radar.
Nor would it have been their last from what the cops learned in talking with them about this abduction. They were 'predators' and hunted down opportunities to abduct children. I suspect they will find other children they have abducted along the way. There home was in the country so I imagine the police will be searching the property very carefully.