The robbery that you refer to occurs in Chapter 4, after Dunstan's plan of selling the horse belonging to Godfrey have come to naught when the horse dies in an accident. Making his way through Raveloe at dark, he comes across Marner's house, and is planning on intimidating Silas Marner into giving him some money and letting him gain interest from it as another alternative source of money. As he gets closer and enters the house, he is surprised to see that Silas Marner has abandoned his home with food cooking and the lantern on. Dunstan, perhaps looking for justification for the crime he is about to commit, wonders if Silas Marner had gone out to get more fuel and slipped into the Stone-pit:
That was an interesting idea to Dunstan, carrying consequences of entire novelty. If the weaver was dead, who had a right to his money? Who would know where his money was hidden? Who would know that anybody had come to take it away?
With this thought, Dunstan determines to find the weaver's fabled hoard and make off with it himself.