i'm crunching a few numbers here, assuming it was low 20's C at 5 pm and dropped to high single digits by midnight, and using newton's law of cooling. ... if he had died at 5 pm, 6 hours would be just at the edge of still being detectable. if he had died somewhere in between, he would have been warmer for longer, and more visible to the IR. and if he was still alive, he would have been easily detectable, even if hypothermic.
as to whether the family would have known possible holes etc. ... i'm sure they would know many, especially close to the house and places they pass regularly, but not all. and i think there's two factors at play here that are easy to underestimate: 1) how much work it is to search a big area, and 2) how easy it is to overlook a body, especially a small one. as you may know, there have been plenty of cases where a body was found in an area that had previously been searched.
to be clear, i'm not wedded to the belief that he is concealed somewhere far from house, only that IF he's out there, he's concealed. the more they don't find him away from the house, the more i think he's somewhere near it.