Welcome WWW&H.
It is possible that William had a spiderman toy in his hand when he was playing outside, and it became lodged somewhere in the van ... to be discovered when the van was impounded, searched, and forensically tested. It may have been one of those details that the police hold back, when they are searching for the correct perpetrator.
There was an article (linked below) that showed a pic of William's foster parents with a bag of his toys. They clearly showed a spiderman toy among these toys (I always wondered why they specifically showed that toy ... perhaps it was a bait for the perpetrator, a quiet 'signal' that they knew about him). William may have had several spiderman dolls, just like my daughter had several Barbie dolls when she was small.
All the while William's parents clutch a bagful of his favourite toys, including a Spiderman doll.
http://www.dailymail.co.uk/news/art...sightings-reported-three-times-day-place.html
4 September 2015