Avery had 3 days to put the car there before she was even reported missing and was on his own property. The police had just over a day to put the car there after she was reported missing, onto someone else's property while trying to keep it quiet from the media, the Avery's and the volunteer searchers that were around the property. Not only did they manage to do that, they managed to displace the two cars that were next to it, hide the licence plates, disconnect the battery, and cover the car with branches which would have taken more time and a lot more risk. That's arguably without knowing she was even dead.