I'm sure this has been seen before [i think it was Jean Brunelin who may have drew our attention to it first] but here is a photo of an aspect of the Fleury tableau. Just underneath the 'pink' building perhaps Sauniere was telling us there is a tunnel system - because quite clearly there are tunnels depicted. It kind of reminds me [the depiction of the tunnels] of Cherisey's admonition in the last chapter of Circuit - Charlot has crawled into a narrow passage and a rock crevice .... "At the end of a fairly brief journey, but one that seems unending, there is a fork where he must make a choice .... cellis or arcis? Right or left, lets go for the left one and 'Long live the King'!
It does look like Sauniere, if he painted in these details or requested the details to be added, might have been trying to suggest, that not far from the 'pink' building is some kind of hole in the ground which leads to a long tunnel deep underground - which branches off in two direction's. I believe Brunelin thought he could see a coffin in the 'close-up's' too.
Updated 6th December 2013:
I knew i had read about this cave complex before - here is the link HERE ! It was Monsieur Brunelin who first identified this aspect on the Fleury tableau - and from this webpage i hope they do not mind me cross-posting the picture Brunelin seems to have taken:
Welcome to the blog of Rhedesium
My name is Sandy Hamblett, inspired and passionate researcher of the mysteries at Rennes-les-Bains.