No, septic tanks are well sealed, otherwise properties with septics would smell all the time, and that is not the case. Even the immediate neighbours - if they had septic too, and depending on the type of lid, etc.. if he wandered only a little and into an immediate neighbour's yard, it is not inconceivable for small children to run and flip the lid, fall in, and the lid flip right back on, as if nothing ever happened.
A long time ago I posted an article about that exact thing happening, and in that particular case, if the child's dad had not happened to see it for himself, it never would have been suspected to have happened. But this is all old conversation (as is most of what of the conversation on this thread, it seems), it just still bothers me a lot to think that this simple thing wasn't done for William, but yet there seem to be enough resources to have a troop 50 strong, looking in brush for 4 weeks straight, with the expectation of finding nothing, apparently.