"[Reuben](rc://en/tw/dict/bible/names/reuben) returned to the pit, and was surprised to see that Joseph was not there." The word "behold" here shows that [Reuben](rc://en/tw/dict/bible/names/reuben) was surprised to find out Joseph was gone.
This is an act of deep distress and grief. This can be written more clearly. AT: "He was so grieved that he tore his clothes" (See: [[rc://en/ta/man/translate/figs-explicit]])