I think that the scenario in terms of time is the same regardless of whether it is a lurker or someone who was just driving past, as in, I was including both in my statement. They had to happen to be in exactly the right spot when William emerged around the house, with absolutely no guarantees there would be this opportunity. They could have turned up at any time - 4 hours beforehand, or 5 minutes beforehand - but there was apparently one, and only one, opportunity to get him. Then they still had to act with the same speed as anyone else.
I'm not sure someone who was lurking would have been there for ages. The longer they are there the more chance they have of being seen. Plus prior to that they would have seen the kids being thoroughly supervised outside. But anyway, that's really beside the point. I don't think the likelihood of one scenario is greater or less than any of the others when looking at just the time available. Except the missing one - if he'd simply wandered off I think the time to do that and not be seen or ultimately found makes it more unlikely than the others.