If you turn right at the end of the road the carrier was seen on you are a very short distance from the OC and a body could be concealed along the way in a bin or abandoned building. Remember the searchers were looking for a live child, not a dead body.
It can't be a bin, because according to Amaral the body was collected 3 weeks later - and the bins would have been emptied long before then. I would have thought 'abandoned buildings' would be the obvious places to look - especially by the locals who were searching.
The idea that the McCanns, (in the full glare of reporters - all of whom were desperate for 'copy'), would even entertain the idea of getting into their car to go down the road, park up, - retrieve the body from a building in PdL, then carry it out to the car, drive off somewhere else and then remove it from the boot, hide/bury it, then get back into their car and calmly drive home is too ludicrous for words. They would have to be stark staring mad to even think about doing that.
The man the Smith's saw wasn't Gerry - it was a physical impossibility - unless he was wearing a bright blue suit with a big yellow 'S' on the front and red underpants over his tights. As far as I know no such outfit has been discovered in PdL....