You're right in all you say of course. Although, there are still a lot of unexplored areas above, below, to the sides and between the tunnels and the shaft that its quite feasible that a very small error by the original builders could have placed Sil slightly off-centre or just outside the excavated parts. The chance is there at least, certainly sufficient to make any claim that he isn't there mighty dodgy.
And that's assuming they wanted to place him centrally of course. Considering we haven't the faintest idea what the monument was for that's a pretty unjustified assumption for us to make. EH are forever telling us what a miniscule proportion of the whole volume the voided portions are. All we know is that we haven't found him in a tiny proportion of one percent of the Hill. It makes no sense to say that there's no burial in a monument that remains 99.7% totally unexplored - especially as at the same time we're marvelling that it is so unlike the norm and it is surrounded by hundreds of smaller green lumps every one of which had a burial in it. Not to mention being within site of a number of Long Barrows. No sign of burials in those being placed centrally is there?
