I think you might be referring to media articles that were written a little unclearly, saying that the dogs hadn't found Lucas and Lucas hadn't been found either inside the house or outside the house. That made people think that the dogs hadn't found a scent of Lucas ever having been in the house. But I think what was meant was that neither the dogs sniffing outside, nor the human LE searchers searching inside the house and garden had actually found *Lucas* (not referring to his scent or DNA but the actual Lucas).
I believe they take something that smells of the missing person and present it to the dog at the nearest place where the person was last sighted, and if that was inside a house they present it to the dog on the doorstep outside the door that was reportedly left open, for example. I don't think they try and trace the person's movements within the house with a sniffer dog. A cadaver dog would be used inside if there's suspicion that a person may not have left the house alive, but LE have never clarified, to my knowledge, whether cadaver dogs were taken into the house.
