Hugo starts this chapter with a bit of a folk tale about people believing that the devil would bury his wealth in the forest outside Montfermeil. This superstition followed that if you approached the devil it would turn out to be a farmer and you would die within a week. If you waited and dug up what he buried you would die within a month. If you walked away and acted like nothing happened you would die in a year.
Building off of this, Hugo tells us that during the few days before Jean Valjean escaped and was caught again, he had visited the district of Montfermeil. Around this same time a man named Boulatruelle was seen repeatedly going off into the woods with his shovel, apparently trying to dig something up.
At first the towns people mistook him for the devil, then they decided it was just Boulatruelle who they inferred must have seen the devil bury some of his treasure and was out trying to find it!
Eventually Boulatruelle stopped this behavior, but by then the word of what he was doing spread throughout Montfermeil and our old nemesis Thenardier took an interest in it.
The story goes on that Thenardier got Boulatruelle drunk with the intention of finding out what was happening. Boulatruelle told the story of seeing a very large man who he said he knew from the past (Thenardier assumed from prison) go out into the woods to bury a chest. It was too dark to follow and he had been unable to find the chest himself.
As the reader it is clear that this man was Jean Valjean, but we are given no indication that Thenardier knew this. So here we have another tenuous connection to Thenardier that further shows him to be someone always out to lay his hands on some kind of wealth he didn’t work for, while also letting us know that Jena Valjean had been to Montfermeil and had stashed his money there.
This opens several unanswered questions: Did he see Cosette? Did he confront Thenardier? Why did he hide the money there of all places? I guess we have to read on to find out!