A possible explanation of what happened given some aspects of what the security services have done (remove the door, not entered the house even to feed the pets) and compatible with the timing of the Skripals leaving the house and getting sick a long time later, this is totally speculative:
* The daughter brings from Russia some toxic substance as the Skripals are into making some small black market profits.
* The father examines the merchandise but he is not carefu enough and contaminates his hands a very little bit.
* They leave the house, he opens and closed the house door, leaving most of the contamination on the door handle.
* While walking or chatting he touches the daughter with the contaminated hand.
* When they collapse on the bench the policeman helps them to stand by taking their hand.
If the above entirely speculative scenario is true then:
* The *inner* handle of the door is also contaminated.
* There is/was something fairly dangerous inside the house and that’s why it was sealed.
* The toxic substance is not particularly toxic or volatile.
* It would also explain why the toxic substance was not washed away by the rain and why nobody was so stupid to put it on the external handle exposed to the elements.
* it would also explain why the OPCW has been careful to emphasize that the toxic substance is very pure, as it came from a bottle or something, not exposed to pollen or dust or fog or rain on an external door handle.
There are several possible variants…
It would be really nice to know if the inner handle has been examined, but of course we are very unlikely to ever be told. It would be interesting to see what will happen in court. With the fantastic density of CCTV in english towns and 5 weeks having passed I guess the suspects have already been figured out
:-)