Does anyone know whether William was wearing footwear when he vanished? I can see he is barefoot on the decking.
Also, I am a great believer in the skills of trained dogs, having read a lot about them during the McCann case, so I have a slight issue with the term 'fallible' when referring to the dogs. As I see it, they alert to the scent they are sniffing for if they locate it, and they don't alert if they don't locate that scent. So when we read that the dogs did not alert, that doesn't mean they have failed, it just means that the scent was not located - it may have not been imprinted on surroundings. I am no expert on this but I recall reading that textured things 'hold' scents better than some non-textured things, so tree bark may hold a scent better than a smooth metal garage door, for instance. I recall reading that a child in a pushchair/buggy could leave a trackable scent if moving next to a shrubby hedge, given certain weather conditions in the duration between event and tracking attempt. Apart from the trees, the immediate area around the house does not have a lot of texture to retain the scent. The grass is short, there are no wooden gates that William would have to touch and open in order to run a complete lap of the house - in fact he would probably not need to touch a thing on his journey. 'Chaseys' suggests fast, unlingering play - less imprint time.
Also, looking at the picture of William, he seemed to be wearing synthetic blend pyjamas (I have 11years experience in the clothing industry); maybe fibre 'shedding' (that would contain his scent) is minimised as the fibres are smooth and move over each other with less friction than with 100% natural fibres.
The one thing we do 'know' is that William was on that decking that morning for that photo. It makes no sense to me that the dogs would not pick up his scent at that exact spot that we see his skin in contact with. I guess we have just not been 'told' if they did or didn't.
It's all a mystery to me, though I still firmly believe that the young fella was taken from the immediate area, and did not wander off.
Jmo.