‘And Reuben returned to the cistern, and behold, Joseph was not in the cistern, and he tore his clothes, and he returned to his brothers and said, “The child is not, and as for me, where shall I go?
Reuben has quite clearly been away for some unknown reason. It may that he was sickened by their desire for blood and wanted to be on his own, or it may be that as the eldest he went to keep an eye on the sheep while the traders were passing. It was not unknown for a few sheep to disappear when a caravan passed by. But afterwards he goes privately to the cistern which his brothers have now left, in order to release Joseph. However, to his shock and dismay he finds that Joseph has gone. The tearing of clothes was a sign of great distress especially expressing sorrow and grief.
He comes to his brothers and expresses his dismay. He clearly feels he has the responsibility for Joseph's welfare as his eldest brother. What is he going to say to Jacob? Then they no doubt told him what they had done.