In Genesis 37:21-22, Reuben talked his brothers out of killing Joseph:
But when Reuben heard it, he rescued him out of their hands, saying, “Let us not take his life.” And Reuben said to them, “Shed no blood; throw him into this pit here in the wilderness, but do not lay a hand on him”—that he might rescue him out of their hand to restore him to his father.
However, while he was absent, the brothers sell Joseph into slavery. In verses 29-30, Reuben returns to find Joseph gone:
When Reuben returned to the pit and saw that Joseph was not in the pit, he tore his clothes and returned to his brothers and said, “The boy is gone, and I, where shall I go?”
Strictly speaking, the text doesn't say exactly what the brothers told him (beyond Reuben's apparent participation in the cover-up).
In Genesis 42:22, Reuben appears to believe that Joseph was dead:
And Reuben answered them, “Did I not tell you not to sin against the boy? But you did not listen. So now there comes a reckoning for his blood.”
Did he believe all along that the brothers killed him themselves, or merely that he had died as a result of their actions? If he was implying that the brothers had killed him, why didn't the brothers tell him otherwise? Or does the fact that the brothers don't correct him suggest that they thought that he had probably died as a result of their actions too?